Spindle-shaped goethite particles and process for making spindle-shaped goethite particles

ABSTRACT

Spindle-shaped goethite particles of the present invention contain Co of 8 to 45 atm %, calculated as Co, based on whole Fe, Al of 5 to 20 atm %, calculated as Al, based on whole Fe, and have an average major axial diameter of 0.05 to 0.18 μm, each of said spindle-shaped goethite particles comprising a seed portion and a surface layer portion, the weight ratio of said seed portion to said surface layer portion being 30:70 to 80:20 and the relationship of the Co concentration of the seed portion with that of the hematite particle being 50 to 95:100 when the Co concentration of the hematite particle is 100, and said Al existing in said surface layer portion. Such spindle-shaped goethite particles are fine particles and exhibit a good particle size distribution.  
     Spindle-shaped hematite particles obtained form the spindle-shaped goethite particles, can be prevented as highly as possible from causing destruction of particle shape when subjected to a heat-reduction step for producing magnetic spindle-shaped metal particles and magnetic spindle-shaped metal particles containing iron as a main component produced from the spindle-shaped goethite particles or the spindle-shaped hematite particles as a starting material, exhibit a high coercive force, an excellent particle coercive force distribution, a large saturation magnetization and an excellent oxidation stability, and are excellent in a squareness (Br/Bm) of the sheet due to a good dispersibility in a binder resin.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to spindle-shaped goethiteparticles, spindle-shaped hematite particles and magnetic spindle-shapedmetal particles containing iron as a main component. More particularly,the present invention relates to spindle-shaped goethite particles whichare fine particles and exhibit a good particle size distribution(standard deviation/average major axial diameter); spindle-shapedhematite particles which can be prevented as highly as possible fromcausing destruction of particle shape when subjected to a heat-reductionstep for producing magnetic metal particles, and which are suitable as astarting material for the production of spindle-shape magnetic metalparticles containing iron as a main component exhibiting a high coerciveforce, a large saturation magnetization, an excellent oxidationstability and an excellent coercive force distribution (switching fielddistribution) when incorporated into a magnetic coating film(hereinafter sometimes referred to merely as “SFD” or “sheet SFD”); andthe magnetic spindle-shaped metal particles containing iron as a maincomponent which are produced from the spindle-shaped goethite particlesor the spindle-shaped hematite particles as a starting material, whichexhibit a high coercive force, an excellent particle coercive forcedistribution (switching field distribution) (hereinafter referred to asmerely “SFDr” or “particle SFDr”), a large saturation magnetization andan excellent oxidation stability, and which are excellent in asquareness (Br/Bm) of the sheet due to a good dispersibility in a binderresin.

[0002] In recent years, miniaturization, lightening, recording-timeprolongation, high density recording and high storage capacity ofrecording and reproducing apparatuses for audio, video or computers,have proceeded more remarkably. With this progress, magnetic recordingmedia such as magnetic tapes and magnetic discs have been increasinglyrequired to have a high performance and a high recording density.

[0003] Magnetic recording media have been required to show a high imagequality, high output characteristics, and especially improved frequencycharacteristics. For this reason, it has been demanded to enhance aresidual magnetic flux density (Br) and a coercive force of the magneticrecording media.

[0004] These characteristics of the magnetic recording media have aclose relation to the magnetic particles used therefor. In recent years,magnetic metal particles containing iron as a main component haveattracted attention because such particles can show a higher coerciveforce and a larger saturation magnetization as compared to those ofconventional magnetic iron oxide particles, and have been put intopractice and applied to magnetic recording media such as digital audiotapes (DAT), 8-mm video tapes, Hi-8 tapes, video floppies or W-VHS tapesfor Hi-vision. Further, the magnetic metal particles containing iron asa main component have been adopted in DVC system for digital recording,Zip or super-discs for computers, and recently, large-capacity Hi-FDwhich are being now industrially put into practice.

[0005] In consequence, it has also been strongly demanded to furtherimprove properties of these magnetic metal particles containing iron asa main component.

[0006] As to the relationship between various characteristics of themagnetic recording media and properties of the magnetic particles usedtherefor, in order to achieve high density recording, it is generallyrequired that the magnetic particles are fine particles and have a goodparticle size distribution.

[0007] In order to obtain a high image quality, the magnetic recordingmedia for video are required to have a high coercive force (Hc) and alarge residual magnetic flux density (Br). In order to impart such ahigh coercive force (Hc) and a large residual magnetic flux density (Br)to the magnetic recording media, the magnetic particles used thereforare also required to have a coercive force (Hc) as high as possible, anexcellent particle coercive force distribution (SFDr) and a largesaturation magnetization.

[0008] For example, in Japanese Patent Application Laid-Open (KOKAI) No.63-26821 (1988), it is described that “FIG. 1 shows a relationshipbetween the SFD measured on the magnetic disc and the reproductionoutput thereof. . . . As is apparent from FIG. 1, the characteristiccurve representing the relationship between the SFD and the reproductionoutput becomes linear. Therefore, it is recognized that the reproductionoutput of the magnetic disc can be increased by using ferromagneticparticles having a small SFD. Namely, in order to obtain a highreproduction output, it is preferred that the SFD is small, and forexample, when it is intended to obtain a more reproduction output thanordinary one, the SFD is required to be not more than 0.6.” Thus, inorder to enhance the reproduction output of magnetic recording media, itis necessary that the SFD (Switching Field Distribution) of the magneticrecording media is small, i.e., the sheet coercive force distribution ofthe magnetic recording media is narrow. Further, for this purpose, it isrequired that the magnetic particles used therefor has a good particlesize distribution and contain no dendritic particles therein.

[0009] As to the magnetic metal particles containing iron as a maincomponent, the finer the particle size thereof becomes, the larger thesurface activity thereof becomes, so that the magnetic properties isconsiderably deteriorated even in air, because such fine particlesreadily undergo the oxidation reaction by oxygen therein. As a result,it is not possible to produce magnetic metal particles containing ironas a main component, which can show the aimed high coercive force andlarge saturation magnetization.

[0010] In consequence, it has been required to provide magnetic metalparticles containing iron as a main component which are excellent inoxidation stability.

[0011] As described above, at present, there has been a strongest demandfor providing magnetic metal particles containing iron as a mainparticles which are fine particles, contain no dendritic particles, andhave a good particle size distribution, a high coercive force, anexcellent particle coercive force distribution (SFDr), a largesaturation magnetization and an excellent oxidation stability.

[0012] On the other hand, in the production of magnetic recording media,when the magnetic metal particles containing iron as a main componentbecomes finer or have a larger saturation magnetization, there tends tobe caused such a problem that the particles show a poor dispersibilitydue to the increase in attraction force between particles or magneticcohesive force when kneaded and dispersed in a binder resin in anorganic solvent. As a result, the magnetic recording media producedtherefrom tend to be deteriorated in magnetic characteristics,especially squareness (Br/Bm). Therefore, it have been required that themagnetic metal particles are further improved in magnetic properties.

[0013] In general, the magnetic metal particles containing iron as amain component can be produced by using as starting particles, goethiteparticles, hematite particles obtained by heat-dehydrating the goethiteparticles, or particles obtained by incorporating different kind ofmetals other than iron into these particles; heat-treating the startingparticles, if necessary, in a non-reducing atmosphere; and heat-reducingthe thus-treated particles in a reducing gas atmosphere. It is knownthat the obtained magnetic metal particles containing iron as a maincomponent have a similar shape to that of goethite particles as thestarting particles. Therefore, in order to obtain magnetic metalparticles containing iron as a main component which satisfy the abovevarious properties, it is necessary to use goethite particles which arefine particles, have a good particle size distribution and anappropriate particle shape, and contain no dendritic particles. Further,it is required to retain the appropriate particle shape and the goodparticle size distribution of the goethite particles during and afterthe subsequent heat-treatment.

[0014] Conventionally, there are known various methods of producinggoethite particles as starting particles for the magnetic metalparticles containing iron as a main component. As methods ofpreliminarily adding metal compounds containing cobalt which can enhancemagnetic properties, aluminum which can impart a good shape-retentionproperty to the magnetic metal particles due to anti-sintering effectthereof, or the like, during the production of goethite particles, thereare known, for example, (i) a method of passing an oxygen-containing gasthrough a suspension containing ferrous hydroxide colloid obtained byadding not more than one equivalent of an aqueous alkali hydroxidesolution to an aqueous ferrous salt solution in the presence of a cobaltcompound, at a temperature of 50_C. so as to conduct the oxidationreaction, thereby producing acicular goethite particles, followed byconducting a growth reaction thereof (Japanese Patent ApplicationLaid-Open (KOKAI) No. 7-11310 (1995)); (ii) a method of reacting anaqueous ferrous salt solution to which an acid salt compound of aluminumis added, with an aqueous alkali carbonate solution to which a base saltcompound of aluminum is added, thereby obtaining an FeCO₃-containingsuspension, and passing an oxygen-containing gas through the obtainedsuspension so as to conduct the oxidation reaction, thereby producingspindle-shaped goethite particles (Japanese Patent Application Laid-Open(KOKAI) No. 6-228614 (1994)); (iii) a method of neutralizing andhydrolyzing a mixed aqueous solution containing a ferric salt and acobalt compound with an aqueous alkali hydroxide solution so as toobtain goethite seed crystal particles, and subjecting the obtainedgoethite seed crystal particles to growth reaction due to the hydrolysiscaused by neutralizing the alkali hydroxide in an aqueous ferric saltsolution containing an Al compound (Japanese Patent ApplicationLaid-Open (KOKAI) No. 58-176902 (1983)); (iv) a method of aging asuspension containing an Fe²⁺-containing precipitate obtained byreacting an aqueous alkali carbonate with an aqueous ferrous saltsolution, in a non-oxidative atmosphere, and passing anoxygen-containing gas through the suspension so as to conduct theoxidation reaction, thereby producing spindle-shaped goethite particles,wherein a Co compound is preliminarily allowed to exist in either theaqueous ferrous salt solution, the suspension containing anFe²⁺-containing precipitate or the aged suspension containing anFe²⁺-containing precipitate before the oxidation reaction, and whereinan aqueous solution containing a compound of at least one elementselected from the group consisting of Al, Si, Ca, Mg, Ba, Sr, Nd and thelike, is added in a total amount of 0.1 to 5.0 mol %, calculated aselement(s), based on Fe²⁺ in the aqueous ferrous salt solution, in thecourse of the oxidation reaction that the percentage of oxidation ofFe²⁺ therein lies in the range of 50 to 90%, under the same conditionsas those of the oxidation reaction (Japanese Patent ApplicationLaid-Open (KOKAI) No. 7-126704 (1995)); (v) a method of preliminarilyadding Si, a rare earth element or the like during the production ofgoethite particles and then adding a Co compound, and further adding anAl compound in an amount of 6 atm % at most in the course of theoxidation reaction (Japanese Patent Application Laid-Open (KOKAI) Nos.8-165501 (1996) and 8-165117 (1996)); (vi) a method of neutralizingferrous salt with alkali hydroxide and/or alkali carbonate, doping arare earth element and an alkali earth element into iron oxide hydroxideparticles in the vicinity of a surface thereof during the oxidationreaction, and then modifying hydroxides of Al and/or Si on a surface ofthe obtained iron oxide hydroxide particles (Japanese Patent ApplicationLaid-Open (KOKAI) No. 6-140222 (1994)); or the like.

[0015] In addition, as to the oxidation rate upon the production ofgoethite particles, there are known a method of producing goethiteparticles by adjusting an air-flow linear velocity to the specific range(Japanese Patent Application Laid-Open (KOKAI) No. 59-23922 (1984)); amethod of initially oxidizing not less than 30 mol % of whole Fe at thespecific oxidation rate and then oxidizing the remainder of Fe at alarger oxidation rate than the initial oxidation rate but not more thantwo times the initial oxidation rate (Japanese Patent ApplicationLaid-Open (KOKAI) No. 1-212232 (1989)); or the like.

[0016] In the above-mentioned Japanese KOKAIs, there has also beendescribed magnetic metal particles containing iron as a main component,which are produced from goethite particles as starting particles.

[0017] Magnetic metal particles presently strongly demanded are magneticspindle-shaped metal particles containing iron as a main component,which are fine particles, show a good particle size distribution;contain no dendritic particles; have an appropriate particle shape, ahigh coercive force, an excellent particle coercive force distribution(SFDr), a large saturation magnetization and an excellent oxidationstability; and are excellent in sheet squareness (Br/Bm) due to the gooddispersibility in a binder resin. However, in case of using as startingparticles, the goethite particles described in the above-mentionedJapanese KOKAIs, the obtained magnetic metal particles cannotsufficiently satisfy the requirements of these properties.

[0018] That is, in the production method described in Japanese PatentApplication Laid-Open (KOKAI) No. 7-11310 (1995), there can be obtainedacicular goethite particles containing Co therein. However, the goethiteparticles also contain unsuitable dendritic particles therein. Inaddition, the obtained goethite particles cannot necessarily show auniform particle size. Further, it is difficult to obtain a largesaturation magnetization and a high coercive force, due to contents ofCo and Al and positions at which Co and Al exist.

[0019] In the production process described in Japanese PatentApplication Laid-Open (KOKAI) No. 6-228614 (1994), goethite particleswhich are free from inclusion of dendritic particles and have a uniformparticle size, are produced by appropriately controlling the addition ofaluminum. However, since the Al content is 6 atm % at most (calculatedas Al) based on Fe and the surface of each goethite particle is coatedwith a Co compound, it is difficult to obtain a large saturationmagnetization and a high coercive force.

[0020] In the production process described in Japanese PatentApplication Laid-Open (KOKAI) No. 7-126704 (1995), the Co compound isadded in an amount of 1 to 8 atm %, and further the Al compound is addedin an amount of 5 atm % at most in the course of the oxidation reaction.However, it is difficult to obtain magnetic metal particles containingiron as a main component, which show a high coercive force, a largesaturation magnetization and an excellent oxidation stability.

[0021] In the production processes described in Japanese PatentApplication Laid-Open (KOKAI) Nos. 8-165501 (1996) and 8-165117 (1996),since the amount of aluminum added is 6 atm % at most, it is difficultto obtain magnetic metal particles containing iron as a main component,which have a high coercive force, a large saturation magnetization andan excellent oxidation stability, and further the dispersibility in abinder resin is considered to be poor. Meanwhile when the Al compound isadded in the course of the oxidation reaction, it is required tocontinue the oxidation reaction under the same conditions as those ofthe initial stage.

[0022] In the production process described in Japanese PatentApplication Laid-Open (KOKAI) No. 58-176902 (1983), since Fe³⁺ is usedas a starting material, the reaction mechanism is not oxidation buthydrolysis, and further the hydrothermal treatment (autclavingtreatment) as a second-reaction is conducted at a temperature as high asmore than 100_C.

[0023] In the production process described in Japanese PatentApplication Laid-Open (KOKAI) No. 6-140222 (1994), no Co is added,thereby failing to obtain magnetic metal particles showing a largesaturation magnetization and an excellent oxidation stability.

[0024] In Japanese Patent Application Laid-Open (KOKAI) No. 59-23922(1984), there is no description that Al, Co, etc., which are effectivefor sintering prevention, exist in the goethite particles in the form ofa solid solution, nor description that the linear velocity of theoxygen-containing gas is increased in the course of the oxidationreaction.

[0025] The production process described in Japanese Patent ApplicationLaid-Open (KOKAI) No. 1-212232 (1989), aims at conducting anindustrially advantageous process in a short time. In order to attainthe aim, after not less than 30 mol % of whole Fe is initially oxidized,the oxidation rate is increased in order to oxidize the remainder of Fe.However, since the oxidation rate is less than two times the initialrate, it is still insufficient to attain the aim. In addition, in thespecification thereof, there is no description that Co and Al which areeffective for sintering prevention and for imparting good magneticproperties to resultant magnetic metal particles, are contained ingoethite particles.

[0026] Further, it is hardly said that the magnetic metal particlesproduced from the goethite particles as starting particles obtainedaccording to the process described in the above Japanese KOKAIs, arefine particles which show a good particle size distribution, contain nodendritic particles, have a high coercive force, an excellent particlecoercive force distribution (SFDr), a large saturation magnetization, anexcellent oxidation stability and a good dispersibility in a binderresin, and are excellent in sheet squareness (Br/Bm) due to the gooddispersibility.

[0027] On the other hand, in order to obtain magnetic recording mediahaving a higher coercive force, an excellent coercive force distribution(SFD) and an excellent weather resistance (ΔBm), it has been stronglyrequired that the magnetic metal particles containing iron as a maincomponent have not only a higher coercive force and a larger saturationmagnetization, but also a particle size distribution as narrow aspossible, an excellent dispersibility in vehicle and an excellentoxidation stability (Δσs).

[0028] However, in any of these conventional processes, it is difficultto obtain magnetic metal particles which can fulfill the aboverequirements of various properties.

[0029] As described above, the magnetic metal particles containing ironas a main component can be produced by using spindle-shaped goethiteproduced by conducting the oxidation reaction by passing anoxygen-containing gas through an aqueous solution containing anFe-containing precipitate obtained by reacting an aqueous ferrous saltsolution with an aqueous alkali solution, spindle-shaped hematiteparticles obtained by heat-dehydrating the thus obtained goethiteparticles, or particles obtained by incorporating different kind ofmetals other than iron into the spindle-shaped hematite particles, asstarting particles; and heat-reducing the starting particles in areducing gas atmosphere.

[0030] Since the conditions used in the heat-reduction step such asatmosphere, temperature, etc., are extremely severe, the sintering tendsto be caused within or between the spindle-shaped hematite particles.Especially, in order to obtain a large saturation magnetization which isone advantage of the magnetic metal particles, it is required to controlthe heat-reducing temperature to as high a level as possible, so as toproceed the reduction reaction to a sufficient extent. However, when theheat-reducing temperature is increased, there is a tendency that thespindle-shaped hematite particles undergo destruction of particle shape.

[0031] Alternatively, in order to obtain a high coercive force, it isrequired that the magnetic metal particles are smaller in particle sizeand, therefore, the spindle-shaped hematite particles used as startingparticles thereof are also required to have a fine particle size.However, in the case of fine particles having a particle size of notmore than 0.15 μm, the destruction of particle shape in theheat-reduction step tends to be caused more remarkably. The magneticmetal particles in which the particle shape is destroyed, cannot show ahigh coercive force due to poor anisotropy in particle shape, so thatthe particle size distribution thereof is deteriorated. In the casewhere such fine particles are used for the production of magneticrecording media, the dispersibility of these particles in vehicle isdeteriorated due to the increase in attraction force between theparticles or the increase in magnetic cohesive force when kneaded anddispersed in the vehicle, resulting in deterioration in squareness(Br/Bm) as a magnetic coating film and, therefore, failing to obtainmagnetic recording media having an excellent SFD.

[0032] In consequence, it is strongly demanded to provide spindle-shapedhematite particles which can be prevented as highly as possible frombeing destroyed in particle shape when subjected to the heat-reductionstep.

[0033] Further, when such spindle-shaped fine magnetic metal particlescontaining iron as a main component, especially those having a majoraxial diameter of not more than 0.15 μm, are taken out and placed in airafter the heat-reduction step, the oxidation reaction of these particlesproceeds drastically by oxygen in air, resulting in considerabledeterioration in magnetic properties thereof, especially in saturationmagnetization thereof. As a result, the aimed magnetic metal particleshaving a large saturation magnetization cannot be obtained, and further,when these particles are used to form a magnetic coating film, theweather resistance (ΔBm) of the coating film is deteriorated. Therefore,it is also strongly demanded to provide magnetic metal particles showingnot only a large saturation magnetization even immediately after theheat-reduction step, but also an excellent oxidation stability.

[0034] Hitherto, in order to improve the oxidation stability of magneticmetal particles containing iron as a main component, there is widelyknown a method of incorporating Co as a different element other than Fein an amount as large as more than 20 atm % (Japanese Patent ApplicationLaid-Open (KOKAI) Nos. 3-174704 (1991), 3-293703 (1991), 5-101917(1993), 6-176912 (1994), 9-22522 (1997), 9-22523 (1997), etc.). Further,as the method of reducing a particle size of magnetic metal particlescontaining iron as a main component which show a high coercive force,there is known a method of producing fine magnetic metal particlescontaining iron as a main component (Japanese Patent ApplicationLaid-Open (KOKAI) No. 57-135436 (1982)).

[0035] Although starting particles presently demanded are spindle-shapedCo-containing hematite particles which can be prevented as highly aspossible from being destroyed in particle shape in the heat-reductionstep, there cannot be still obtained such starting particles which canfulfill the above properties.

[0036] Namely, in the method of reducing a particle size of finemagnetic metal particles containing iron as a main component asdescribed above, since fine spindle-shaped hematite particles are usedas starting particles, the sintering tends to be caused therewithinand/or therebetween upon the heat-reduction, resulting in destruction ofthe particle shape of the spindle-shaped hematite particles. For thisreason, it is difficult to obtain magnetic metal particles having theaimed high coercive force. The coercive force of the magnetic metalparticles obtained by the above method is 2,000 Oe at most. Further, thedestruction of particle shape upon the heat-reduction, results in poordispersibility in vehicle and deterioration in SFD as a magnetic coatingfilm.

[0037] In the case where Co is added in a large amount, there can beobtained magnetic metal particles which are improved in oxidationstability. However, upon the heat-treatment, excessive growth ofparticles tends to occur, thereby inducing the destruction of particleshape. As a result, since the obtained magnetic metal particles aredeteriorated in anisotropy of particle shape, it is not possible toobtain a high coercive force. Further, since the magnetic metalparticles are deteriorated in particle size distribution anddispersibility in vehicle, the SFD of a magnetic coating film is about0.40 at most.

[0038] As a result of the present inventorsí earnest studies for solvingthe above problems, it has been found that in a process of producingspindle shaped goethite particles which process comprises reacting amixed aqueous alkali solution comprising an aqueous alkali carbonatesolution and an aqueous alkali hydroxide solution, with an aqueousferrous salt solution to obtain a water suspension containing anFe²⁺-containing precipitate; aging the water suspension containing theFe²⁺-containing precipitate in a non-oxidative atmosphere; passing anoxygen-containing gas through the resultant water suspension to conductthe oxidation reaction, thereby producing spindle-shaped goethite seedcrystal particles; and passing an oxygen-containing gas through a watersuspension containing both the Fe²⁺-containing precipitate and thespindle-shaped goethite seed crystal particles to conduct the oxidationreaction, thereby growing a goethite layer on a surface of eachspindle-shaped goethite seed crystal particle,

[0039] upon the production of the spindle-shaped goethite seed crystalparticles, by adding a Co compound in an amount of 8 to 45 atm %(calculated as Co) based on whole Fe, to the water suspension containingthe Fe²⁺-containing precipitate during the aging-treatment beforeinitiation of the oxidation reaction, and conducting the oxidationreaction to oxidize 30 to 80 mol % of whole Fe²⁺, and

[0040] upon the growth of the goethite layer, by adjusting a linearvelocity of the oxygen-containing gas passed through the watersuspension containing both the Fe²⁺-containing precipitate and thespindle-shaped goethite seed crystal particles, to not less than twotimes that of the oxygen-containing gas passed through the watersuspension containing the Fe²⁺-containing precipitate upon theproduction of the goethite seed crystal particles, and adding an Alcompound in an amount of 5 to 20 atm % (calculated as Al) based on wholeFe,

[0041] there can be obtained spindle-shaped goethite particles whichcontain 8 to 45 atm % of Co (calculated as Co) based on whole Fe and 5to 20 atm % of Al (calculated as Al) based on whole Fe; which have anaverage major axial diameter of 0.05 to 0.18 μm; and which comprise aseed portion and a surface layer portion, wherein the weight ratio ofthe seed portion to the surface layer portion is 30:70 to 80:20, the Coconcentration of the seed portion is less than that of the surface layerportion, and Al exists only in the surface layer portion; and furtherwhich are fine particles, have an excellent particle size distribution(standard deviation/major axial diameter) and an appropriate particleshape, and are free from inclusion of dendritic particles. The presentinvention has been attained on the basis of the finding.

SUMMARY OF THE INVENTION

[0042] It is an object of the present invention to providespindle-shaped goethite particles which are fine particles and free frominclusion of dendritic particles, and have a good particle sizedistribution and an appropriate particle shape.

[0043] It is an another object of the present invention to providespindle-shaped hematite particles suitable as starting particles for theproduction of the magnetic spindle-shaped metal particles containing Feas a main component which can be prevented as highly as possible frombeing destroyed in particle shape upon the heat-reduction step, and showa higher coercive force, especially not less than 2,000 Oe, a largesaturation magnetization, especially not less than 130 emu/g, anexcellent oxidation stability, and an excellent SFD of a magneticcoating film, especially less than 0.40.

[0044] It is further object of the present invention to provide magneticspindle-shaped metal particles containing iron as a main component whichare produced from the spindle-shaped goethite particles orspindle-shaped hematite particles as starting particles, show a highcoercive force, an excellent particle coercive force distribution(SFDr), a large saturation magnetization and an excellent oxidationstability, and are excellent in sheet squareness (Br/Bm) due to a gooddispersibility in a binder resin.

[0045] To accomplish the aims, in a first aspect of the presentinvention, there is provided spindle-shaped goethite particlescontaining cobalt of 8 to 45 atm %, calculated as Co, based on whole Fe,aluminum of 5 to 20 atm %, calculated as Al, based on whole Fe, andhaving an average major axial diameter of 0.05 to 0.18 μm,

[0046] each of said spindle-shaped goethite particles comprising a seedportion and a surface layer portion, the weight ratio of said seedportion to said surface layer portion being 30:70 to 80:20 and therelationship of the Co concentration of the seed portion with that ofthe goethite particle being 50 to 95:100 when the Co concentration ofthe goethite particle is 100, and the aluminum existing only in saidsurface layer portion.

[0047] In a second aspect of the present invention, there is providedspindle-shaped goethite particles containing cobalt of more than 20 atm% and not more than 45 atm %, calculated as Co, based on whole Fe,aluminum of 5 to 15 atm %, calculated as Al, based on whole Fe, andhaving an average major axial diameter of 0.05 to 0.17 μm, an averageminor axial diameter of 0.010 to 0.025 μm, an aspect ratio (averagemajor axial diameter/average minor axial diameter) of 4:1 to 8:1, and aBET specific surface area of 100 to 250 m²/g, the aluminum existing onlyin said surface layer portion.

[0048] In a third aspect of the present invention, there is providedspindle-shaped hematite particles containing cobalt of 8 to 45 atm %,calculated as Co, based on whole Fe, aluminum of 5 to 20 atm %,calculated as Al, based on whole Fe, and a rare earth element of 1 to 15atm %, calculated as rare earth element, based on whole Fe, and havingan average particle size of 0.05 to 0.17 μm,

[0049] each of said spindle-shaped hematite particles comprising a seedportion, an intermediate layer portion and an outer layer portion, theweight ratio of said seed portion to said intermediate layer portionbeing 30:70 to 80:20 and the relationship of the Co concentration of theseed portion with that of the hematite particle being 50 to 95:100 whenthe Co concentration of the hematite particle is 100, the aluminumexisting only in said intermediate layer portion and said rare earthelement existing in said outer layer portion.

[0050] In a fourth aspect of the present invention, there is providedspindle-shaped hematite particles containing cobalt of more than 20 atm% and not more than 45 atm %, calculated as Co, based on whole Fe,aluminum of 5 to 15 atm %, calculated as Al, based on whole Fe, and arare earth element of 5 to 15 atm %, calculated as rare earth element,based on whole Fe, and having an average major axial diameter of 0.05 to0.14 μm, an aspect ratio (average major axial diameter/average minoraxial diameter) of 4:1 to 8:1, a crystallite size D₁₀₄ of 50 to 80_, asaturation magnetization σs of 0.5 to 2 emu/g, the aluminum existingonly in said intermediate layer portion and said rare earth elementexisting in said outer layer portion.

[0051] In a fifth aspect of the present invention, there is providedmagnetic spindle-shaped metal particles containing iron as a maincomponent, which contain cobalt of 8 to 45 atm %, calculated as Co,based on whole Fe, aluminum of 5 to 20 atm %, calculated as Al, based onwhole Fe, and a rare earth element of 1 to 15 atm %, calculated as rareearth element, based on whole Fe, and have an average major axialdiameter of 0.05 to 0.15 μm.

[0052] In a sixth aspect of the present invention, there is providedmagnetic spindle-shaped metal particles containing iron as a maincomponent, which contain cobalt of more than 20 atm % and not more than45 atm %, calculated as Co, based on whole Fe, aluminum of 5 to 15 atm%, calculated as Al, based on whole Fe, and a rare earth element of 5 to15 atm %, calculated as rare earth element, based on whole Fe, and havean average major axial diameter of 0.05 to 0.14 μm, an aspect ratio(average major axial diameter/average minor axial diameter) of 4:1 to8:1, an X-ray crystallite size D₁₁₀ of 12.0 to 17.0 nm, a coercive forceof 2,000 to 2,500 Oe and a saturation magnetization σs of 130 to 160emu/g.

[0053] In a seventh aspect of the present invention, there is provided aprocess for producing the spindle-shaped goethite particles, comprising:

[0054] aging a water suspension containing an Fe²⁺-containingprecipitate produced by reacting a mixed aqueous alkali solutioncomprising an aqueous alkali carbonate solution and an aqueous alkalihydroxide solution, with an aqueous ferrous salt solution, in anon-oxidative atmosphere;

[0055] conducting an oxidation reaction by passing an oxygen-containinggas through the water suspension, thereby producing spindle-shapedgoethite seed crystal particles; and

[0056] passing again an oxygen-containing gas through the resultantwater suspension containing both said Fe²⁺-containing precipitate andsaid spindle-shaped goethite seed crystal particles to conduct theoxidation reaction of the water suspension, thereby growing a goethitelayer on a surface of each spindle-shaped goethite seed crystalparticle,

[0057] upon the production of said spindle-shaped goethite seed crystalparticles, a Co compound being added in an amount of 8 to 45 atm %,calculated as Co, based on whole Fe, to said water suspension containingthe Fe²⁺-containing precipitate during the aging treatment beforeinitiation of the oxidation reaction, thereby oxidizing 30 to 80% ofwhole Fe²⁺, and

[0058] upon the growth of said goethite layer, a linear velocity of saidoxygen-containing gas passing through said water suspension containingboth the Fe²⁺-containing precipitate and the spindle-shaped goethiteseed crystal particles, being adjusted to not less than two times thatof the oxygen-containing gas passing through the water suspensioncontaining the Fe²⁺-containing precipitate upon the production of thegoethite seed crystal particles, and an Al compound being added in anamount of 5 to 20 atm %, calculated as Al, based on whole Fe.

[0059] In an eighth aspect of the present invention, there is provided aprocess for producing spindle-shaped hematite particles, comprising:

[0060] treating said spindle-shaped goethite particles obtained in theseventh aspect with an anti-sintering agent comprising a rare earthelement-containing compound; and

[0061] heat-treating the spindle-shaped goethite particles at 400 to850_C. in a non-reducing atmosphere.

[0062] In a ninth aspect of the present invention, there is provided aprocess for producing magnetic spindle-shaped metal particles containingiron as a main component, comprising:

[0063] treating said spindle-shaped goethite particles obtained in theseventh aspect with an anti-sintering agent comprising a rare earthelement-containing compound; and

[0064] then heat-reducing said spindle-shaped goethite particles at 400to 700_C. in a reducing atmosphere.

[0065] In a tenth aspect of the present invention, there is provided aprocess for producing magnetic spindle-shaped metal particles containingiron as a main component, comprising:

[0066] treating said spindle-shaped goethite particles obtained in theseventh aspect with an anti-sintering agent comprising a rare earthelement-containing compound;

[0067] heat-treating the treated spindle-shaped goethite particles at400 to 850_C. in a non-reducing atmosphere; and

[0068] then heat-reducing said heat-treated particles at 400 to 700_C.in a reducing atmosphere.

[0069] In an eleventh aspect of the present invention, there is provideda process for producing magnetic spindle-shaped metal particlescontaining iron as a main component, comprising:

[0070] heat-reducing said spindle-shaped hematite particles obtained inthe eighth aspect at 400 to 700_C. in a reducing gas atmosphere.

[0071] In a twelfth aspect of the present invention, there is provided aprocess for producing magnetic spindle-shaped metal particles containingiron as a main component, which are suitable for magnetic recording,comprising:

[0072] charging spindle-shaped goethite particles containing cobalt of20 to 45 atm %, calculated as Co, based on whole Fe and having a majoraxial diameter of 0.05 to 0.15 μm, or spindle-shaped hematite particlesobtained by heat-dehydrating said goethite particles, as startingparticles, into a fixed-bed reducing apparatus to form a fixed-bedhaving a height of not more than 30 cm;

[0073] elevating the temperature of said starting particles to 400 to700_C. in an inert gas atmosphere;

[0074] replacing the inert gas atmosphere with a reducing gasatmosphere; and

[0075] reducing said spindle-shaped goethite particles or spindle-shapedhematite particles with a reducing gas fed at a linear velocity of 40 to150 cm/s, at temperature of 400 to 700_C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0076]FIG. 1 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles obtained in Example1 according to the present invention.

[0077]FIG. 2 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained in Example2 according to the present invention.

[0078]FIG. 3 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles containingiron as a main component which were obtained in Example 3 according tothe present invention.

[0079]FIG. 4 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles obtained in Example9 according to the present invention.

[0080]FIG. 5 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained in Example16 according to the present invention.

[0081]FIG. 6 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles containingiron as a main component which were obtained in Example 22 according tothe present invention.

[0082]FIG. 7 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles obtained inComparative Example 1.

[0083]FIG. 8 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles containingiron as a main component which were obtained in Comparative Example 10.

[0084]FIG. 9 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles obtained in Example4 according to the present invention.

[0085]FIG. 10 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained in Example5 according to the present invention.

[0086]FIG. 11 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles containingiron as a main component which were obtained in Example 6 according tothe present invention.

[0087]FIG. 12 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles 1 as startingparticles.

[0088]FIG. 13 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles 2 as startingparticles.

[0089]FIG. 14 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped goethite particles 3 as startingparticles.

[0090]FIG. 15 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained in Example30 according to the present invention.

[0091]FIG. 16 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained in Example31 according to the present invention.

[0092]FIG. 17 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained inComparative Example 15.

[0093]FIG. 18 is a transmission electron micrograph (×30,000) showing aparticle shape of spindle-shaped hematite particles obtained inComparative Example 17.

[0094]FIG. 19 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles obtained inExample 34 which were produced from spindle-shaped hematite particlesobtained in Example 30.

[0095]FIG. 20 is a transmission electron micrograph (×30,000) showing aparticle shape of magnetic spindle-shaped metal particles obtained inExample 35 which were produced from spindle-shaped hematite particlesobtained in Example 31.

[0096]FIG. 21 shows an X-ray diffraction pattern (mark ∘: hematite, andmark ?: spinel-type iron oxide) of spindle-shaped hematite particlesobtained in Example 5.

[0097]FIG. 22 shows an X-ray diffraction pattern (mark ∘: hematite, andmark ?: spinel-type iron oxide) of spindle-shaped hematite particlesobtained in Comparative Example 15.

[0098]FIG. 23 shows an X-ray diffraction pattern (mark ∘: hematite) ofspindle-shaped hematite particles obtained in Comparative Example 16.

DETAILED DESCRIPTION OF THE INVENTION

[0099] The present invention is described in detail below.

[0100] First, the spindle-shaped goethite particles according to thepresent invention are explained.

[0101] The spindle-shaped goethite particles according to the presentinvention have an average major axial diameter of usually 0.05 to 0.18μm, preferably 0.05 to 0.17 μm, more preferably 0.05 to 0.16 μm, stillmore preferably 0.05 to 0.15 μm, a particle size distribution (standarddeviation/average major axial diameter) of usually not more than 0.24,preferably 0.10 to 0.24, more preferably 0.10 to 0.22, an average minoraxial diameter of 0.010 to 0.025 μm, preferably 0.010 to 0.023 μm, andan aspect ratio (average major axial diameter/average minor axialdiameter) of usually 4:1 to 8:1, preferably 4:1 to 7.7:1.

[0102] The BET specific surface area of the spindle-shaped goethiteparticles according to the present invention, is usually 100 to 250m²/g, preferably 120 to 230 m²/g.

[0103] The spindle-shaped goethite particles according to the presentinvention contain cobalt in an amount of usually 8 to 45 atm %,preferably 10 to 40 atm % (calculated as Co) based on whole Fe, andaluminum in an amount of usually 5 to 20 atm %, preferably 6 to 15 atm %(calculated as Al) based on whole Fe.

[0104] Each particle of the spindle-shaped goethite particles accordingto the present invention, comprises a goethite seed portion (coreparticle) and a goethite surface layer portion.

[0105] The seed portion means a goethite seed crystal particle formed byoxidizing the ferrous salt added, before adding the Al compound. Morespecifically, the seed portion represents a portion extending outwardlyfrom the center of each particle and having a specific weight percentagedetermined according to a percentage of oxidation of Fe²⁺. The weightpercentage of the seed portion is usually 30 to 80% by weight,preferably 40 to 70% by weight based on the weight of the goethiteparticle from the center of each particle.

[0106] The Co concentration of the seed portion of the goethite particleis represented by the relationship of the Co concentration of the seedportion of the goethite particle with that of the goethite particle. TheCo concentration of the seed portion of the goethite particle isexpressed by the Co content (atm %) in the seed portion of the goethiteparticle based on the Fe content in the seed portion of the goethiteparticle, and the Co concentration of the goethite particle is expressedby the whole Co content (atm %) in the goethite particle based on thewhole Fe content in the goethite particle. The relationship of the Coconcentration of the seed portion with that of the goethite particle isusually 50 to 95:100, preferably 60 to 90:100, when the Co concentrationof the goethite particle is 100. If the relationship of the Coconcentration of the seed portion with that of the goethite particle isless than 50:100, the effect of improving magnetic properties of themagnetic particles produced from the goethite particle may not beobtained. On the other hand, if the relationship of the Co concentrationis more than 95:100, it is difficult to exhibit a sufficientshape-retention property upon the reduction reaction, thereby causingdeterioration in magnetic properties thereof.

[0107] The surface layer portion means a goethite layer formed by thegrowth reaction on the surface of each goethite seed crystal particleafter the addition of the Al compound. More specifically, the surfacelayer portion represents a portion extending inwardly from an outersurface of each goethite particle. The weight percentage of the surfacelayer portion is usually 20 to 70% by weight, preferably 30 to 60% byweight based on the weight of the goethite particle from the outersurface of each particle.

[0108] The Co concentration in the surface layer portion of the goethiteparticle is more than that of the seed portion of the goethite particle.The Co concentration of the surface layer portion of the goethiteparticle is represented by the relationship of the Co concentration ofthe surface layer portion of the goethite particle with that of thegoethite particle. The Co concentration of the surface layer portion ofthe goethite particle is expressed by the Co content (atm %) in thesurface layer portion of the goethite particle based on the Fe contentin the surface layer portion of the goethite particle, and the Coconcentration of the goethite particle is expressed by the whole Cocontent (atm %) in the goethite particle based on the whole Fe contentin the goethite particle. The relationship of the Co concentration ofthe surface layer portion with that of the goethite particle is usually102 to 300:100, preferably 106 to 194:100, when the Co concentration ofthe goethite particle is 100.

[0109] Aluminum is present only in the surface layer portion, and the Alcontent is usually 5 to 20 atm %, preferably 6 to 15 atm %, morepreferably 7 to 12 atm % (calculated as Al) based on whole Fe. When theAl content is less than 5 atm %, the anti-sintering effect may not beobtained. On the other hand, when the Al content is more than 20 atm %,the obtained goethite particles may be deteriorated in magneticproperties, especially saturation magnetization.

[0110] The X-ray crystallite size ratio (D₀₂₀/D₁₁₀) of thespindle-shaped goethite particles according to the present invention, isnot less than 2.0:1, preferably 2.0 to 3.0.

[0111] Next, the process for producing the spindle-shaped goethiteparticles according to the present invention is described.

[0112] The spindle-shaped goethite particles according to the presentinvention can be obtained by producing goethite seed crystal particlesand then growing a goethite layer on the surface of each goethite seedcrystal particle.

[0113] The goethite seed crystal particles can be obtained by aging awater suspension containing an Fe²⁺-containing precipitate produced byreacting a mixed aqueous alkali solution composed of an aqueous alkalicarbonate solution and an aqueous alkali hydroxide solution, with anaqueous ferrous salt solution, in a non-oxidative atmosphere, andpassing an oxygen-containing gas through the water suspension to formspindle-shaped goethite seed crystal particles. Upon the production ofthe goethite seed crystal particles, a Co compound can be added in anamount of usually 8 to 45 atm %, preferably 10 to 40 atm % (calculatedas Co) based on whole Fe, to the water suspension containing theFe²⁺-containing precipitate during the aging treatment before theinitiation of the oxidation reaction.

[0114] The aging of the water suspension is preferably conducted at atemperature of usually 40 to 80_C. in a non-oxidative atmosphere. Whenthe temperature is less than 40_C., the obtained goethite particles mayhave a small aspect ratio, so that the aging effect may not besufficiently exhibited. On the other hand, when the temperature is morethan 80_C., magnetite particles tend to be contained in the obtainedgoethite particles. The aging time is usually 30 to 300 minutes,preferably 60 to 300 minutes. When the aging time is less than 30minutes, it is difficult to sufficiently increase the aspect ratio ofthe goethite particles. The aging time may be more than 300 minutes, butsuch a long aging time cannot show a further improvement and, therefore,meaningless.

[0115] The non-oxidative atmosphere can be obtained by passing an inertgas such as a nitrogen gas, or a reducing gas such as a hydrogen gas,through a reactor into which the water suspension is accommodated.

[0116] In the production reaction of the spindle-shaped goethite seedcrystal particles, as the aqueous ferrous salt solution, there may beused an aqueous ferrous sulfate solution, an aqueous ferrous chloridesolution or the like.

[0117] The mixed aqueous alkali solution used in the production reactionof the spindle-shaped goethite seed crystal particles, can be obtainedby mixing an aqueous alkali carbonate solution with an aqueous alkalihydroxide solution. As to the mixing ratio (expressed by % calculated asnormality), the percentage of the aqueous alkali hydroxide solution isusually 10 to 40%, preferably 15 to 35% (% calculated as normality).When the percentage of the aqueous alkali hydroxide solution is lessthan 10%, the aspect ratio of the obtained goethite seed crystalparticles may be unsatisfactory. On the other hand, when the percentageof the aqueous alkali hydroxide solution is more than 40%, magnetiteparticles tend to be contained in the obtained goethite particles.

[0118] As the aqueous alkali carbonate solutions, there may be used anaqueous sodium carbonate solution, an aqueous potassium carbonatesolution, an aqueous ammonium carbonate solution, or the like. As theaqueous alkali hydroxide solutions, there may be used am aqueous sodiumhydroxide solution, potassium hydroxide solution or the like. As theaqueous alkali hydroxide solution, there may be used an aqueous sodiumhydroxide solution, an aqueous potassium hydroxide solution, or thelike.

[0119] The equivalent ratio of the mixed aqueous alkali solution towhole Fe in the aqueous ferrous salt solution is usually 1.3:1 to 3.5:1,preferably 1.5:1 to 2.5:1. When the equivalent ratio of the mixedaqueous alkali solution to whole Fe is less than 1.3;1, magnetiteparticles tend to be contained in the obtained goethite particles. Onthe other hand, when the equivalent ratio thereof is more than 3.5:1,the amount of the mixed aqueous alkali solution becomes large, which isdisadvantageous from the industrial viewpoint.

[0120] The Fe²⁺ concentration after mixing the aqueous ferrous saltsolution with the mixed aqueous alkali solution, is usually 0.1 to 1.0mol/liter, preferably 0.2 to 0.8 mol/liter. When the Fe²⁺ concentrationis less than 0.1 mol/liter, the yield of goethite particles becomes low,which is disadvantageous from the industrial viewpoint. On the otherhand, when he Fe²⁺ concentration is more than 1.0 mol/liter, theparticle size distribution of the obtained goethite particles becomesdisadvantageously large.

[0121] The pH value in the production reaction of the spindle-shapedgoethite seed crystal particles, is usually 8.0 to 11.5, preferably 8.5to 11.0. When the pH value is less than 8.0, a large amount of acidradicals tend to be contained in the obtained goethite particles. Sincesuch acid radicals cannot be simply removed even by washing, thesintering between particles is caused when magnetic metal particlescontaining iron as a main component are produced from these goethiteparticles. On the other hand, when the pH value is more than 11.5, it isdifficult to obtain magnetic metal particles having the aimed highcoercive force.

[0122] The production reaction of the spindle-shaped goethite seedcrystal particles may be conducted by the oxidation reaction by passingan oxygen-containing gas (e.g., air) through the solution.

[0123] The linear velocity of the oxygen-containing gas is usually 0.5to 3.5 cm/s, preferably 1.0 to 3.0 cm/s.

[0124] The linear velocity means an amount of the oxygen-containing gaspassed per a unit sectional area (a bottom sectional area of a columnreactor, and a diameter and number of pores of a bottom plate are nottaken into consideration), and is expressed by a unit of cm/second.

[0125] The temperature used for the production reaction of thespindle-shaped goethite seed crystal particles, is usually not more than80_C. at which goethite particles are produced. When the temperature ismore than 80_C., magnetite particles tend to be contained in theobtained spindle-shaped goethite particles. The temperature thereof ispreferably 45 to 55_C.

[0126] In the production reaction of the spindle-shaped goethite seedcrystal particles, as the Co compounds, there may be used cobaltsulfate, cobalt chloride, cobalt nitrate or the like. The Co compoundmay be added to the water suspension containing the Fe²⁺-containingprecipitate during the aging treatment thereof before the initiation ofthe oxidation reaction.

[0127] The amount of the Co compound added is usually 8 to 45 atm %,preferably 10 to 40 atm %, more preferably 10 to 35 atm % (calculated asCo) based on whole Fe. When the amount of the Co compound added is lessthan 8 atm %, the effect of improving magnetic properties of the finallyproduced magnetic metal particles may not be exhibited. On the otherhand, when amount of the Co compound added is more than 45 atm %, theobtained particles are too fine to exhibit a small aspect ratio.

[0128] In the growth reaction of the goethite layer, the pH valuethereof is usually 8.0 to 11.5, preferably 8.5 to 11.0. When the pHvalue is less than 8.0, a large amount of acid radicals may be containedin the obtained goethite particles. Since the acid radicals may not besimply removed even by washing, the sintered particles tend to be causedwhen the magnetic metal particles are produced from these goethiteparticles. On the other hand, when the pH value is more than 11.5, therecannot be obtained magnetic metal particles having the aimed highcoercive force.

[0129] The growth reaction of the goethite layer may be conducted by theoxidation reaction by passing an oxygen-containing gas (e.g., air)through the solution.

[0130] In the growth reaction of goethite particles, the linear velocityof the oxygen-containing gas passed upon the growth reaction, is usuallynot less than two times, preferably 2 to 3.5 times that in theproduction reaction of the seed crystal particles. When the linearvelocity is less than two times, the viscosity of the water suspensionmay be increased upon the addition of Al, so that the crystal growth inthe minor axial direction may be accelerated and, therefore, the aspectratio of the obtained particles may be lowered. More specifically, thelinear velocity of the oxygen-containing gas passed upon the growthreaction, is usually 1.0 to 7.0 cm/s, preferably 2.0 to 6.0 cm/s.

[0131] The temperature used in the growth reaction of the goethitelayer, is usually not more than 80_C. at which goethite particles areproduced. When the temperature is more than 80_C., magnetite particlestends to be contained in the obtained goethite particles. Thetemperature thereof is preferably 45 to 55_C.

[0132] In the growth reaction of the goethite layer, as the Alcompounds, there may be used acid salts such as aluminum sulfate,aluminum chloride or aluminum nitrate; or aluminates such as sodiumaluminate, potassium aluminate or ammonium aluminate; or the like.

[0133] The Al compound may be added (i) simultaneously when the linearvelocity of the oxygen-containing gas reaches not less than two timesthat upon the production reaction of the seed crystal particles, (ii)during or (iii) after passing the oxygen-containing gas at the linearvelocity of not less than two times that upon the production reaction ofthe seed crystal particles. The addition of the Al compound ispreferably conducted (i) simultaneously when the linear velocity of theoxygen-containing gas reaches not less than two times that upon theproduction reaction of the seed crystal particles, or (ii) duringpassing the oxygen-containing gas at the linear velocity of not lessthan two times that upon the production reaction of the seed crystalparticles. When the Al compound is added in parts, continuously orintermittently, the effect of the present invention may not besufficiently exhibited.

[0134] The amount of the Al compound added is usually 5 to 20 atm %,preferably 6 to 15 atm %, more preferably 7 to 12 atm % (calculated asAl) based on whole Fe in the spindle-shaped goethite particles as afinal product. When the amount of the Al compound added is less than 5atm %, the anti-sintering effect may not be obtained. On the other hand,when the amount of the Al compound added is more than 20 atm %,particles other than goethite particles may be produced, so that themagnetic properties of the obtained particles, especially saturationmagnetization, may be deteriorated.

[0135] Incidentally, prior to the growth reaction of the goethite layer,the obtained goethite seed crystal particles may be aged in anon-oxidative atmosphere before passing the oxygen-containing gas at thelinear velocity thereof reaches not less than two times that upon theproduction reaction of the seed crystal particles. In such a case, theaging may be conducted under the same conditions as those used in theaging treatment conducted before the production reaction of the goethiteseed crystal particles.

[0136] Next, the spindle-shaped hematite particles according to thepresent invention are described.

[0137] The spindle-shaped hematite particles according to the presentinvention has an average major axial diameter of usually 0.05 to 0.17μm, preferably 0.05 to 0.15 μm, more preferably 0.05 to 0.14 μm, stillmore preferably 0.05 to 0.13 μm, a particle size distribution (standarddeviation/average major axial diameter) of usually not more than 0.22,preferably 0.10 to 0.22, more preferably 0.10 to 0.20, an average minoraxial diameter of usually 0.010 to 0.025 μm, preferably 0.010 to 0.023μm, more preferably 0.010 to 0.022 μm, still more preferably 0.010 to0.020 μm, and an aspect ratio (average major axial diameter/averageminor axial diameter) of 4:1 to 8:1, preferably 4:1 to 7.5:1.

[0138] The BET specific surface area of the spindle-shaped hematiteparticles according to the present invention, is usually 30 to 150 m²/g,preferably 50 to 120 m²/g.

[0139] The spindle-shaped hematite particles according to the presentinvention contain cobalt in an amount of usually 8 to 45 atm %,preferably 10 to 40 atm %, more preferably 10 to 35 atm % (calculated asCo) based on whole Fe, aluminum in an amount of usually 5 to 20 atm %,preferably 6 to 15 atm %, more preferably 7 to 12 atm % (calculated asAl) based on whole Fe, and a rare earth element in an amount ofpreferably 1 to 15 atm %, more preferably 4 to 12 atm %, still morepreferably 5 to 10 atm % (calculated as rare earth element) based onwhole Fe.

[0140] As the suitable rare earth elements, there may be used at leastone rare earth element selected from the group consisting of scandium,yttrium, lanthanum, cerium, praseodymium, neodymium and samarium. Amongthem, yttrium and neodymium are preferable.

[0141] The spindle-shaped hematite particles according to the presentinvention, comprise a hematite seed portion (core particle), a hematiteintermediate layer portion formed on the surface portion of the coreparticle, and an outer layer portion formed on the surface portion ofthe intermediate layer portion.

[0142] The seed portion of the spindle-shape hematite particles is aportion which is derived from the seed portion of the starting goethiteparticles. The seed portion is a portion which extends outwardly fromthe center of each particle. The weight percentage of such a seedportion is usually 30 to 80% by weight, preferably 40 to 70% by weightbased on the weight of the seed portion and intermediate layer portionof the hematite particle.

[0143] The Co concentration of the seed portion of the hematite particleis represented by the relationship of the Co concentration of the seedportion of the hematite particle with that of the hematite particle. TheCo concentration of the seed portion of the hematite particle isexpressed by the Co content (atm %) in the seed portion of the hematiteparticle based on the Fe content in the seed portion of the hematiteparticle, and the Co concentration of the hematite particle is expressedby the whole Co content (atm %) in the hematite particle based on thewhole Fe content in the hematite particle. The relationship of the Coconcentration of the seed portion with that of the hematite particle isusually 50 to 95:100, preferably 60 to 90:100, when the Co concentrationof the hematite particle is 100.

[0144] The intermediate layer portion is which is derived from thesurface layer portion of the starting goethite particles. Theintermediate layer portion is a portion which extends inwardly from anouter surface of each particle excluding the outer layer portioncomposed of rare earth element. The weight percentage of theintermediate layer portion, is usually 20 to 70% by weight, preferably30 to 60% by weight based on the weight of the seed portion andintermediate layer portion of the hematite particle.

[0145] The Co concentration in the intermediate layer portion of thehematite particle is more than that of the seed portion of the hematiteparticle. The Co concentration of the intermediate layer portion of thehematite particle is represented by the relationship of the Coconcentration of the intermediate layer portion of the hematite particlewith that of the hematite particle. The Co concentration of theintermediate layer portion of the hematite particle is expressed by theCo content (atm %) in the intermediate layer portion of the hematiteparticle based on the Fe content in the intermediate layer portion ofthe hematite particle, and the Co concentration of the hematite particleis expressed by the whole Co content (atm %) in the hematite particlebased on the whole Fe content in the hematite particle. The relationshipof the Co concentration of the intermediate layer portion with that ofthe hematite particle is usually 102 to 300:100, preferably 106 to194:100, when the Co concentration of the hematite particle is 100.

[0146] Aluminum may exist only in the intermediate layer portion, andthe Al content is usually 5 to 20 atm %, preferably 6 to 15 atm %, morepreferably 7 to 12 atm % (calculated as Al) based on whole Fe. When theAl content is less than 5 atm %, the anti-sintering effect may not beobtained. On the other hand, when the Al content is more than 20 atm %,the obtained hematite particles are deteriorated in magnetic properties,especially saturation magnetization.

[0147] The outer layer portion may be composed of compounds of rareearth element.

[0148] The content of rare earth element in the outer layer portion, ispreferably 1 to 15 atm %, more preferably 4 to 12 atm %, still morepreferably 5 to 10 atm % (calculated as the rare earth element) based onwhole Fe. When the content of rare earth element in the outer layerportion is less than 1 atm %, the anti-sintering effect may not beobtained. On the other hand, when the content of rare earth element ismore than 15 atm %, the saturation magnetization of the obtainedhematite particles may be reduced.

[0149] Next, the process for producing the spindle-shaped hematiteparticles according to the present invention is described.

[0150] The spindle-shaped goethite particles used as starting particlesmay be coated with an anti-sintering agent to impart an anti-sinteringproperty thereto, in advance of the heat-dehydration treatment.

[0151] As the anti-sintering agent, there may be used compounds of rareearth elements.

[0152] As the suitable compounds of rare earth elements, there may beexemplified compounds containing at least one rare earth elementselected from the group consisting of scandium, yttrium, lanthanum,cerium, praseodymium, neodymium, samarium and the like. Examples of thecompounds of rare earth elements may include chlorides, sulfates,nitrates, etc., of the above-mentioned rare earth elements. As themethod of coating the goethite particles with the compounds of rareearth elements, there may be used either a dry-coating method or awet-coating method. Among them, the wet-coating method is preferable.

[0153] The amount of the compound of the rare earth element used ispreferably 1 to 15 atm %, more preferably 4 to 12 atm %, still morepreferably 5 to 10 atm % (calculated as the rare earth element) based onwhole Fe. When the amount of the compound of the rare earth element usedis less than 1 atm %, a sufficient anti-sintering effect may not beobtained, so that when magnetic metal particles are produced from suchhematite particles, the particle coercive force distribution (SFDr)thereof may be deteriorated. On the other hand, when the amount of thecompound of the rare earth element used is more than 15 atm %, thesaturation magnetization of the obtained particles may be lowered.

[0154] In order to enhance the anti-sintering effect, there may be usedone or more compounds containing at least one element selected from thegroup consisting of Al, Si, B, Ca, Mg, Ba and Sr, if necessary. Thesecompounds not only can exhibit the anti-sintering effect, but also cancontrol the reduction rate. Therefore, these compounds may be used incombination depending upon requirements. In the case where the compoundscontaining the other elements are used in combination with the rareearth compounds, the total amount of the rare earth compounds as theanti-sintering agent and the compounds containing the other elementsthan the rare earth element is preferably 1 to 15 atm % (calculated asthe sum of respective elements) based on whole Fe in the spindle-shapedgoethite particles. When the total amount is too small, a sufficientanti-sintering effect may not be obtained. On the other hand, when thetotal amount is too large, the saturation magnetization of the finallyobtained magnetic metal particles may be deteriorated. Accordingly, theamounts of the rare earth compounds and the compounds of the otherelements used may be appropriately selected according to the combinationthereof to obtain an optimum effect.

[0155] By preliminarily coating the spindle-shaped goethite particleswith the anti-sintering agent or the like, there can be obtainedspindle-shaped hematite particles which can be prevented from causingthe sintering therewithin or therebetween, and can successively maintaina particle shape and aspect ratio of the spindle-shaped goethiteparticles, resulting in facilitating the production of independentmagnetic metal particles containing iron as a main component which canwell maintain the particle shape and the like of the starting goethiteparticles.

[0156] The spindle-shaped hematite particles can be produced byheat-treating the spindle-shaped goethite particles coated with theanti-sintering agent, at a temperature of 400 to 850_C., preferably 450to 800_C. in a non-reducing atmosphere.

[0157] The thus-obtained hematite particles may be washed after theheat-treatment to remove impurity salts such as Na₂SO₄ therefrom. Inthis case, the washing of the hematite particles is preferably conductedunder such a condition that no anti-sintering agent as the coating layeris eluted out and only unnecessary impurity salts can be removedtherefrom.

[0158] More specifically, in order to effectively remove cationicimpurities, the pH value of the wash water is increased, while in orderto effectively remove anionic impurities, the pH value of the wash wateris decreased.

[0159] The particle shape of the starting spindle-shaped goethiteparticles is successively maintained, and the spindle-shaped hematiteparticles according to the present invention has a particle shapegradually tapered from the thick center portion to opposite ends,contain no dendritic particles and, therefore, can show an excellentparticle size distribution.

[0160] The spindle-shaped hematite particles of second embodiment of thepresent invention, contain cobalt of usually more than 20 atm % and notmore than 45 atm %, preferably 21 to 40 atm %, more preferably 21 to 35atm % (calculated as Co) based on whole Fe, aluminum of usually 5 to 15atm %, preferably 6 to 14 atm % (calculated as Al) based on whole Fe,and a rare earth element of usually 5 to 15 atm %, preferably 5 to 12atm % (calculated as rare earth element) based on whole Fe.

[0161] The spindle-shaped hematite particles of second embodiment in thepresent invention, have an average major axial diameter of preferably0.05 to 0.14 μm, more preferably 0.05 to 0.13 μm.

[0162] The spindle-shaped hematite particles of second embodiment in thepresent invention, have an aspect ratio (average major axialdiameter/average minor axial diameter) of usually 4:1 to 8:1, preferably4:1 to 7.5:1.

[0163] The spindle-shaped hematite particles of second embodiment in thepresent invention, have a crystallite size (D₁₀₄) of preferably 50 to80_, more preferably 50 to 78_.

[0164] When the crystallite size (D₁₀₄) is less than 50_, the crystalgrowth of the spindle-shaped hematite particles may be lowered, so thatcontrol of the reduction rate may be sometimes insufficient. Further,due to destruction of the particle shape, it is sometimes difficult toobtain the aimed high coercive force. On the other hand, when thecrystallite size (D₁₀₄) is more than 80_, due to excessive crystalgrowth of the spindle-shaped hematite particles, the crystal growth inthe minor axial direction may also accelerated, so that theshape-anisotropy tends to be lowered and it is sometimes difficult toobtain a high coercive force.

[0165] The spindle-shaped hematite particles of second embodiment in thepresent invention, have a saturation magnetization (σs) of preferably0.5 to 2 emu/g, more preferably 0.5 to 1.5 emu/g.

[0166] When the saturation magnetization (σs) is less than 0.5 emu/g,the spindle-shaped hematite particles show a good particle sizedistribution, but the magnetic spindle-shaped metal particles producedtherefrom may be sometimes deteriorated in dispersibility in vehicle. Onthe other hand, when the saturation magnetization (σs) is more than 2emu/g, a large amount of spinel compounds may be produced upon the heatdehydration due to the cobalt contained therein, so that the crystalgrowth in the minor axial direction may be caused. Further, in the casewhere the crystal growth becomes excessive, the destruction of particleshape tends to be caused, so that it is sometimes difficult to obtainmagnetic spindle-shaped metal particles having a high coercive force.

[0167] The spindle-shaped hematite particles of second embodiment in thepresent invention, have an average minor axial diameter of preferably0.010 to 0.022 μm, more preferably 0.010 to 0.020 μm.

[0168] The particle size distribution (standard deviation/average majoraxial diameter) of the spindle-shaped hematite of second embodiment inthe present invention, is usually not more than 0.22, preferably 0.10 to0.22, more preferably 0.10 to 0.20, still more preferably 0.10 to 0.19.

[0169] The spindle-shaped hematite particles of second embodiment in thepresent invention, have a BET specific surface area of usually 30 to 150m²/g, preferably 50 to 120 m²/g.

[0170] Next, the magnetic spindle-shaped metal particles containing ironas a main component according to the present invention are described.

[0171] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention, have an average majoraxial diameter of usually 0.05 to 0.15 μm, preferably 0.05 to 0.14 μm,more preferably 0.05 to 0.13 μm, a particle size distribution (standarddeviation/average major axial diameter) of usually not more than 0.20,preferably 0.10 to 0.20, more preferably 0.10 to 0.19, still morepreferably 0.10 to 0.18, and an average minor axial diameter of usually0.010 to 0.022 μm, preferably 0.010 to 0.020 μm, more preferably 0.010to 0.018 μm and an aspect ratio (average major axial diameter/averageminor axial diameter) of usually 4:1 to 8:1, preferably 4:1 to 7.5:1,preferably 4:1 to 7:1.

[0172] The BET specific surface area of the magnetic spindle-shapedmetal particles containing iron as a main component according to thepresent invention, is usually 35 to 65 m²/g, preferably 40 to 60 m²/g.

[0173] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention, may contain cobalt inan amount of usually 8 to 45 atm %, preferably 10 to 40 atm %, morepreferably 10 to 35 atm % (calculated as Co) based on whole Fe, aluminumin an amount of usually 5 to 20 atm %, preferably 6 to 15 atm %, morepreferably 7 to 12 atm % (calculated as Al) based on whole Fe, and arare earth element in an amount of usually 1 to 15 atm %, preferably 4to 12 atm % (calculated as rare earth element) based on whole Fe.

[0174] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention, further have acoercive force of usually 1,800 to 2,500 Oe, preferably 1,900 to 2,500Oe, more preferably 2,000 to 2,500 Oe, preferably 2,200 to 2,500 Oe, anda saturation magnetization σs of usually 110 to 160 emu/g, preferably120 to 160 emu/g, more preferably 130 to 160 emu/g.

[0175] The particle coercive force distribution (SFDr) of the magneticspindle-shaped metal particles according to the present invention whichis obtained from the remanence (DC-erased residual magnetization) curve,is usually not more than 0.72, preferably not more than 0.718.

[0176] The X-ray crystallite size D₁₁₀ of the magnetic spindle-shapedmetal particles according to the present invention, is usually 12.0 to17.0 nm, preferably 13.0 to 16.5 nm, more preferably 13.0 to 16.0 nm.

[0177] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention, further have a change(Δσs) in saturation magnetization (σs) with passage of time of usuallynot more than 15%, preferably not more than 10%, more preferably notmore than 8% (as an absolute value) after being subjected to anaccelerated deterioration test for one week at a temperature of 60_C.and a relative humidity of 90%.

[0178] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention, further have goodsheet characteristics which means magnetic characteristics obtained whenforming a magnetic coating film, more specifically a sheet squareness(Br/Bm) of usually not less than 0.85, preferably not less than 0.86,and a sheet SFD (sheet coercive force distribution) of usually not morethan 0.44, preferably not more than 0.42, preferably not more than 0.40.

[0179] Next, the magnetic spindle-shaped metal particles containing ironas a main component which are produced by using the spindle-shapedhematite particles of the above second embodiment in the presentinvention are described.

[0180] The magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, maycontain cobalt in an amount of usually more than 20 atm % and not morethan 45 atm %, preferably 21 to 40 atm %, more preferably 21 to 35 atm %(calculated as Co) based on whole Fe, aluminum in an amount of usually 5to 15 atm %, preferably 6 to 14 atm % (calculated as Al) based on wholeFe, and a rare earth element in an amount of 5 to 15 atm %, preferably 5to 12 atm % (calculated as rare earth element) based on whole Fe.

[0181] The magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, have anaverage major axial diameter of usually 0.05 to 0.14 μm, preferably 0.05to 0.13 μm, an aspect ratio (average major axial diameter/average minoraxial diameter) of usually 4:1 to 8:1, preferably 4:1 to 7.5:1, an X-raycrystallite size D₁₁₀ of usually 12.0 to 17.0 nm, preferably 13.0 to16.5 nm, a particle size distribution (standard deviation/average majoraxial diameter) of usually not more than 0.18, preferably 0.10 to 0.18,more preferably 0.10 to 0.17, and an average minor axial diameter ofusually 0.010 to 0.020 μm, preferably 0.010 to 0.018 μm.

[0182] The magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, furtherhave a BET specific surface area of usually 35 to 65 m²/g, preferably 40to 60 m²/g.

[0183] The magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, furtherhave a coercive force of usually 2,000 to 2,500 Oe, preferably 2,100 to2,500 Oe, and a saturation magnetization σs of usually 130 to 160 emu/g,preferably 135 to 160 emu/g.

[0184] The magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, furtherhave a change (Δσs) in saturation magnetization σs with passage of time(oxidation stability) of usually not more than 10%, preferably not morethan 8% (as an absolute value) after being subjected to an accelerateddeterioration test for one week at a temperature of 60_C. and a relativehumidity of 90%.

[0185] As to characteristics of the magnetic coating film produced byusing the magnetic spindle-shaped metal particles containing iron as amain component of second embodiment in the present invention, the sheetsquareness (Br/Bm) is usually not less than 0.85, preferably not lessthan 0.86, and the sheet SFD is usually less than 0.40, preferably notmore than 0.39.

[0186] Further, as to the characteristics of the magnetic coating film,the change (ΔBm) in saturation magnetic flux density (Bm) with passageof time which represents a weather resistance of the magnetic coatingfilm after subjected to an accelerated deterioration test for one weekat 60_C. and a relative humidity of 90%, is usually not more than 8%,preferably not more than 6% as an absolute value.

[0187] Next, the process for producing the magnetic spindle-shaped metalparticles containing iron as a main component according to the presentinvention, is described.

[0188] In accordance with the present invention, the magneticspindle-shaped metal particles containing iron as a main component maybe produced either by treating the spindle-shaped goethite particlesaccording to the present invention, with the above anti-sintering agent,and then directly heat-reducing the thus-treated particles, or byheat-reducing the spindle-shaped hematite particles according to thepresent invention.

[0189] Further, the magnetic spindle-shaped metal particles containingiron as a main component may also be produced by continuously subjectingthe spindle-shaped goethite particles treated with the anti-sinteringagent, to heat-treatment in a non-reducing atmosphere and then toheat-reduction in a reducing atmosphere.

[0190] Although the aimed magnetic spindle-shaped metal particlescontaining iron as a main component may be produced by directly reducingthe spindle-shaped goethite particles treated with the anti-sinteringagent, in order to attain well-controlled magnetic properties, particleproperties and particle shape, it is preferred that the spindle-shapegoethite particles treated with the anti-sintering agent arepreliminarily heat-treated in the non-reducing atmosphere by an ordinarymethod in advance of the heat-reduction.

[0191] The non-reducing atmosphere may be formed by a gas flow or a gasstream composed of at least one gas selected from the group consistingof air, an oxygen gas, a nitrogen gas and the like. The heat-treatmenttemperature may be in the range of 400 to 850_C., and it is preferredthat the heat-treatment temperature is appropriately selected dependingupon kind of compounds used for coating the spindle-shaped goethiteparticles. When the heat-treatment temperature is more than 850_C.,deformation of particles or sintering within or between particles tendsto be disadvantageously caused.

[0192] In the process according to the present invention, theheat-reducing temperature is preferably 400 to 700_C. When theheat-reducing temperature is less than 400_C., the reduction reactionproceeds too slowly, so that the process time is disadvantageouslyprolonged. On the other hand, when the heat-reducing temperature is morethan 700_C., the reduction reaction proceeds too rapidly, so that theretend to be caused disadvantages such as deformation of particles orsintering within or between particles.

[0193] The magnetic spindle-shaped metal particles containing iron as amain component according to the present invention which are obtainedafter the heat-reduction, may be taken out in air by known methods,e.g., by a method of immersing in an organic solvent such as toluene; amethod of replacing the atmosphere for the magnetic spindle-shaped metalparticles containing iron as a main component which are produced afterthe heat-reduction, with an inert gas, and then gradually increasing anoxygen content in the atmosphere until the atmosphere is finallyreplaced with air; a method of gradually conducting the oxidation usinga mixed gas composed of oxygen and steam; or the like.

[0194] Alternatively, the magnetic spindle-shaped metal particlescontaining iron as a main component according to the present invention,may be produced by the following method. That is, either spindle-shapedgoethite particles containing cobalt in an amount of usually 20 to 45atm % (calculated as Co) based on whole Fe and having a major axialdiameter of usually 0.05 to 0.15 μm, or spindle-shaped hematiteparticles obtained by heat-dehydrating such spindle-shaped goethiteparticles, are used as starting particles. The starting particles arecharged into a fixed-bed reducing apparatus so as to form a fixed-bedhaving a height of usually not more than 30 cm, preferably 3 to 30 cmtherein. Thereafter, the temperature within the reducing apparatus isincreased to 400 to 700_C. in an inert gas atmosphere. After replacingthe inert gas atmosphere with a reducing gas atmosphere, the startingmaterial are reduced at 400 to 700_C. while passing the reducing gastherethrough at a linear velocity of 40 to 150 cm/s, thereby producingmagnetic spindle-shaped metal particles containing iron as a maincomponent.

[0195] Next, various conditions required for carrying out the presentinvention, are described.

[0196] In the present invention, as starting particles, there may beused the spindle-shaped goethite particles containing cobalt of usually20 to 45 atm % (calculated as Co) based on whole Fe and having anaverage major axial diameter of usually 0.05 to 0.15 μm, or thespindle-shaped hematite particles obtained by heat-dehydrating suchspindle-shaped goethite particles, as described above. Thespindle-shaped hematite particles may contain cobalt of usually 20 to 45atm % (calculated as Co) based whole Fe and have an average major axialdiameter of usually 0.05 to 0.13 μm.

[0197] The starting particles used in the present invention arespindle-shaped particles. The starting spindle-shaped particles containno dendritic particles and show an excellent particle size distribution.

[0198] In the consideration of the anti-sintering effect or the controlof reducing rate, it is preferred that the spindle-shaped goethiteparticles according to the present invention have an average minor axialdiameter of preferably 0.010 to 0.023 μm, an aluminum content ofpreferably 5 to 15 atm % (calculated as Al) based on whole Fe, an aspectratio (average major axial diameter/average minor axial diameter) ofusually 4:1 to 8:1, and a BET specific surface area of usually 100 to250 m²/g.

[0199] The spindle-shaped goethite particles according to the presentinvention, may be coated with Co compounds, Al compounds and theafore-mentioned anti-sintering agents.

[0200] In order to further enhance the anti-sintering effect, one ormore compounds containing at least one other element selected from thegroup consisting of Si, B, Ca, Mg, Ba, Sr and the like, may be added, ifrequired. These compounds show not only the anti-sintering effect, butalso can control the reduction rate. Therefore, these compounds may beused singly or in combination according to the requirements.

[0201] In the consideration of the anti-sintering effect and the controlof reduction rate, it is preferred that the spindle-shaped hematiteparticles according to the present invention, further have an averageminor axial diameter of preferably 0.010 to 0.022 μm, an aluminumcontent of preferably 5 to 15 atm % (calculated as Al) based on wholeFe, a rare earth content of usually 5 to 15 atm % (calculated as rareearth element) based on whole Fe, an aspect ratio (average major axialdiameter/average minor axial diameter) of usually 4:1 to 8:1, and a BETspecific surface area of usually 50 to 120 m²/g.

[0202] The spindle-shaped hematite particles according to the presentinvention, are preferably produced by heat-dehydrating thespindle-shaped goethite particles at 150 to 350_C. in an oxidativeatmosphere, and then heat-treating the obtained particles at atemperature of more than 450_C. and less than 700_C. in the sameatmosphere.

[0203] Further, after the heat-treatment, in order to remove impuritysalts such as Na₂SO₄ which are contained due to the production reactionof the spindle-shaped goethite particles, the spindle-shape hematiteparticles may be washed. In this case, the washing treatment ispreferably conducted under such a condition that no coatinganti-sintering agent is eluted out and only the impurity salts areremoved.

[0204] As the heat-reducing apparatuses usable in the heat-reductionstep, there are known a fluidized bed-type reducing apparatus forheat-reducing a starting material while flowing the material in the formof particles, a fixed bed-type reducing apparatus for heat-reducing afixed bed composed of granules obtained by granulating the startingmaterial.

[0205] With the increased demand for magnetic metal particles, there hasbeen a strong requirement for providing mass-production techniquestherefor. Consequently, the fixed bed-type reducing apparatus isindustrially and economically advantageous, since it is possible tomass-produce magnetic metal particles without scattered particles evenwhen the flow rate of a reducing gas such as hydrogen is increased.

[0206] However, in the case where the heat-reduction is carried out in ahydrogen atmosphere using the fixed bed-type reducing apparatus, a lowerportion of the fixed bed is more rapidly reduced than an upper portionthereof, thereby causing the increase in steam partial pressure.Therefore, the particles in the upper portion of the fixed bed are morelikely to undergo destruction of particle shape and to bring aboutcrystal growth in the minor axial direction, as compared to those in thelower portion. As a result, there is a tendency that properties of theparticles obtained from the lower portion of the fixed bed are differentfrom those of the upper portion.

[0207] On the other hand, by the production process according to thepresent invention using a fixed bed-type reducing apparatus, there isprovided particles obtained from lower and upper portions of the fixedbed in the reducing apparatus can show uniform properties, in which thedestruction of particle shape can be prevented as effectively aspossible.

[0208] In the present invention, before charging into the fixed-bedreducing apparatus, it is preferred that the starting particles aregranulated by an ordinary method to obtain granules having an averageparticle size of usually 1 to 5 mm, preferably 2 to 4 mm.

[0209] As the preferred fixed-bed reducing apparatuses, there may beexemplified a stationary reducing apparatus (batch-type), or a movablereducing apparatus (continuous-type) in which a fixed-bed is formed on amovable belt and reduced while moving the belt.

[0210] In the present invention, the height of the fixed-bed composed ofthe starting particles is usually not more than 30 cm. When the heightis more than 30 cm, the reduction reaction of the fixed-bed may beremarkably accelerated due to a large content of Co. However, a lowerportion of the fixed-bed undergoes too rapid reduction reaction, so thata water-vapor partial pressure may be considerably increased, therebycausing problems such as deterioration in coercive force of an upperportion of the fixed-bed. This results in deteriorating properties ofthe obtained particles as a whole. From the viewpoint of industrialproductivity, the height of the fixed-bed is preferably 3 to 30 cm. Thebatch-type reducing apparatuses (Japanese Patent Application Laid-Open(KOKAI) Nos. 54-62915 (1979) and 4-224609 (1992), etc.) are different inproductivity from continuous-type reducing apparatuses (Japanese PatentApplication Laid-Open (KOKAI) No. 6-93312 (1994), etc.). Therefore, inthe case of the batch-type fixed-bed reducing apparatuses, the height ofthe fixed-bed is preferably more than 8 cm and not more than 30 cm, morepreferably 10 to 30 cm.

[0211] In the process using the fixed-bed reducing apparatuses accordingto the present invention, the atmosphere in which the reducingtemperature is increased to 400 to 700_C., is an inert gas atmosphere.As the preferred inert gases used therefor, there may be exemplified anitrogen gas, a helium gas, an argon gas or the like. Among them, thenitrogen gas is more preferred. In an atmosphere other than the inertgas atmosphere, the particles are reduced during the temperature rise(during which the temperature is changed with passage of time) beforereaching a predetermined reducing temperature. Therefore, since thereducing temperature upon production of the magnetic metal particlesvaries, a uniform crystal growth may not be attained and a high coerciveforce may not be obtained.

[0212] Incidentally, the temperature rise rate is preferably 2 to100_C./min, more preferably 5 to 100_C./min.

[0213] The linear velocity of the inert gas upon the temperature risemay be adjusted such that the granules as the starting particles isprevented from being scattered or destroyed. In the present invention,the linear velocity of the inert gas is preferably 10 to 50 cm/s, morepreferably 20 to 50 cm/s.

[0214] Meanwhile, the method of changing-over the inert gas atmosphereupon the temperature rise to the reducing gas atmosphere upon the heatreduction step, varies depending upon kinds of reducing apparatusesused. From the industrial viewpoint, in the case of the batch-typereducing apparatuses, the changing-over of the atmosphere is preferablyconducted stepwise while controlling an inner pressure thereof, and inthe case of the continuous-type reducing apparatuses, the temperaturerise zone is preferably separated from the reducing zone. In any case,it is preferred that the changing-over is completed in a short time,specifically within 10 minutes.

[0215] The atmosphere used upon the heat-reduction step according to thepresent invention, may be a reducing gas atmosphere. As the reducing gasused for forming the reducing gas atmosphere, hydrogen is preferred.

[0216] The heat-reducing temperature of the heat-reduction step in theprocess using the fixed-bed reducing apparatuses according to thepresent invention, is 400 to 700_C. The heat-reducing temperature can beappropriately selected from the above-mentioned temperature rangeaccording to kind and amount of the compound used for coating thestarting particles. When the heat-reducing temperature is less than400_C., the reduction reaction proceeds too slowly and, therefore, maybe industrially disadvantageous, so that the saturation magnetization ofthe obtained magnetic metal particles may be lowered. On the other hand,when the heat-reducing temperature is more than 700_C., the reductionreaction proceeds too rapidly, so that the destruction of particle shapeor the sintering within or between particles may be caused, resulting indeterioration in coercive force of the obtained particles.

[0217] The linear velocity of the reducing gas used in theheat-reduction step according to the present invention, is 40 to 150cm/s, preferably 50 to 150 cm/s. When the linear velocity of thereducing gas is less than 40 cm/s, the steam produced upon reduction ofthe starting particles may be discharged out of the system too slowly,so that the upper portion of the fixed-bed may be deteriorated incoercive force and SFD, thereby failing to obtain a high coercive forceas a whole. On the other hand, when the linear velocity of the reducinggas is more than 150 cm/s, although the aimed magnetic spindle-shapedmetal particles can be obtained, it may be required to use a higherheat-reducing temperature, or there tend to be caused problems such asscattering and destruction of the granulated material.

[0218] On the other hand, by the production process according to thepresent invention using a fixed bed-type reducing apparatus, there canbe provided magnetic spindle-shaped metal particles containing iron as amain component, having a high coercive force, for example, preferablynot less than 2,100 Oe, and a large saturation magnetization, forexample, preferably not less than 135 emu/g, though the crystallite size(D₁₁₀) thereof is small, especially, preferably not more than 160_.

[0219] The thus obtained magnetic spindle-shaped metal particlescontaining iron as a main component, contain cobalt in an amount ofpreferably 20 to 45 atm %, more preferably 20 to 40 atm %, still morepreferably 20 to 35 atm % (calculated as Co) based on whole Fe, and havean average major axial diameter of preferably 0.05 to 0.12 μm, acoercive force of 2,100 to 2,500 Oe, a saturation magnetization of 135to 160 emu/g and a crystallite size (D₁₁₀) 135 to 160_.

[0220] Meanwhile, with reference to the relationship between crystallitesize and coercive force of known spindle-shaped magnetic metalparticles, in Japanese Patent Application Laid-Open (KOKAI) No. 4-61302(1992), it has been described that ìthere is a tendency that the smallerthe crystallite size, the lower the coercive force. . . . it has beenstrongly demanded to provide magnetic particles capable of exhibiting asmall crystallite size while keeping the coercive force thereof as highas possible. Thus, since the reduction of crystallite size and theincrease of coercive force have a conflicting relationship with eachother, it has been extremely difficult to obtain magnetic metalparticles having a small crystallite size and a high coercive forcesimultaneously.

[0221] In fact, in the method described in the above Japanese PatentApplication Laid-Open (KOKAI) No. 54-62915(1979), the Co content issmall and the linear velocity of gas passed is low, so that the coerciveforce of the obtained magnetic metal particles is as low as about 120Oe. In addition, as shown in Comparative Example hereinafter, since thecoercive force is extremely low and the crystallite size is considerablysmall, properties of the magnetic metal particles are not satisfactory.

[0222] Also, in the method described in the above Japanese PatentApplication Laid-Open (KOKAI) No. 6-93312 (1994), the obtained particlescontain no Co. In addition, as shown in Comparative Example hereinafter,when the atmosphere upon temperature rise is composed of a reducing gas,the coercive force of the obtained magnetic metal particles is as low asabout 1,600 Oe. Further, in comparison with particles obtained inExamples of the present invention as described hereinafter, in the caseof the same crystallite size, the conventional magnetic metal particlesshow a lower coercive force and a smaller saturation magnetization.Therefore, properties of such conventional magnetic metal particles arenot satisfactory.

[0223] In order to improve a particle shape or the like of the goethiteparticles used as starting particles of the magnetic metal particlescontaining iron as a main component, various metal salts may be addedthereto. In this case, cobalt can act for forming a solid solution withFe in the magnetic metal particles produced and, therefore, can enhancethe saturation magnetization and the coercive force Hc thereof, and alsocontribute to enhancement of the oxidation stability. Aluminum canimpart an anti-sintering property to the magnetic metal particlesproduced, and further can impart thereto an excellent shape-retentionproperty and an excellent dispersibility in a binder resin having sodiumsulfonate functional groups, which resin has been ordinarily used in theproduction of magnetic recording media containing magnetic metalparticles.

[0224] It is known that in the case where cobalt exists inside ofgoethite particles, the magnetic metal particles obtained therefrom canshow a larger saturation magnetization as compared to that of theparticles wherein Co exists in an outside portion thereof. It is alsoknown that in the case where the goethite particles are coated with Al,magnetic properties such as coercive force of the magnetic metalparticles obtained therefrom are deteriorated and, therefore, it ispreferred that Al exists in the form of a solid solution in a surfacelayer portion of each particle, thereby enhancing the shape-retentionproperty and the oxidation stability.

[0225] Further, it is also known that in the case where Co is allowed toform a solid solution and both alkali carbonate and alkali hydroxide arejointly used in the production reaction of goethite particles, there canbe obtained fine goethite particles which have a small minor axialdiameter, resulting in an appropriately large aspect ratio. It is alsoknown that Al shows a crystal growth-controlling effect, andconsiderably different aspect ratios are obtained by varying the timingof addition of Al or the amount of Al added. However, there is known nogoethite particles which are fine particles, can show maintain anappropriate aspect ratio and an excellent particle size distribution,and contain a large amount of Co and Al therein.

[0226] Under the circumstances, by dividing the production reaction ofgoethite particles into a seed crystal production reaction and a growthreaction, adding Co showing effects of forming fine particles andappropriately enhancing an aspect ratio, upon the aging treatment beforethe seed crystal production reaction, allowing Co to form a solidsolution such that the Co exists in the goethite seed portion at a lowerconcentration gradient than that in the surface layer portion, passingan oxygen-containing gas upon the growth reaction of the goethite seedcrystal particles at a linear velocity which is not less than two timesthat upon the seed crystal production reaction, and adding Al having ananti-sintering effect simultaneously with or subsequently to the passingstep of the oxygen-containing gas, there can be obtained spindle-shapedgoethite particles which are free from deterioration in aspect ratio dueto the addition of a large amount of Al, are fine particles, can show anappropriate aspect ratio and an excellent particle size distribution,and contain an large amount of Co and Al.

[0227] The reason why the spindle-shaped goethite particles containing alarge amount of Co and Al can be produced, is considered as follows.That is, it has been hitherto considered that although the crystalgrowth in the major axial direction can be appropriately suppressed byadding Al during the growth reaction of the seed crystal particles, theaddition of a large amount of Al causes the increase in viscosity of thewater suspension containing the seed crystal particles, so that therearise problems such as deterioration in aspect ratio and particle sizedistribution due to excessive crystal growth in the minor axialdirection. However, by adjusting the linear velocity of theoxygen-containing gas passed upon the growth reaction to not less thantwo times that upon the seed crystal production reaction, the viscosityof the water suspension containing the seed crystal particles iseffectively reduced, so that it is possible to form a more uniformsurface layer over the surface of each seed crystal particle.

[0228] Further, when the magnetic metal particles containing iron as amain component are produced by subjecting goethite particles toheat-dehydration and then reduction reaction, by using compounds of rareearth element as an anti-sintering agent, there can be obtained magneticspindle-shaped metal particles containing iron as a main component,which are free from inclusion of dendritic particles, and can show notonly an excellent particle size distribution, an appropriate particleshape and an appropriate aspect ratio, but also a high coercive force,an excellent particle coercive force distribution (particle SFDr), alarge saturation magnetization and an excellent oxidation stability.Further, when the obtained magnetic spindle-shaped metal particles and abinder resin having sodium sulfonate functional groups are mixed andformed into a sheet, it is possible to obtain good sheet characteristicssuch as sheet squareness (Br/Bm) and sheet SFD (coercive forcedistribution).

[0229] Alternatively, there can be obtained preferable spindle-shapedhematite particles which have an average major axial diameter of 0.05 to0.14 μm, an aspect ratio of 4:1 to 8:1, a crystallite size (D₁₀₄) of 50to 80_ and a saturation magnetization of 0.5 to 2 emu/g, and containcobalt in an amount of more than 20 atm % and not more than 45 atm %(calculated as Co) based on whole Fe, aluminum in an amount of 5 to 15atm % (calculated as Al) based on whole Fe, and a rare earth compound inan amount of 5 to 15 atm % (calculated as rare earth element) based onwhole Fe. The thus-obtained spindle-shaped hematite particles can beprevented as more effectively as possible from undergoing destruction ofthe particle shape upon the heat-reduction step.

[0230] The reason why the destruction of particle shape can be preventedas more effectively as possible, is considered as follows. That is, itis considered that the merit is attributed to a synergistic effect ofthe suppression of the production of spinel-type iron oxide in thespindle-shaped hematite particles, the an appropriate crystallite size,and the containing of aluminum and rare earth element in specificamounts.

[0231] Further, it is considered that since the spindle-shaped goethiteparticles is heat-dehydrated at not more than 350_C. in an oxidativeatmosphere and then heat-treated at 450 to 700_C. in an oxidative gasatmosphere, the production of spinel-type iron oxide can be effectivelysuppressed and, therefore, the destruction of particle shape can beprevented as effectively as possible, so that it is possible to producespindle-shaped hematite particles having an appropriate crystallite sizeand a good particle size distribution.

[0232] By using such spindle-shaped hematite particles, it is possibleto produce magnetic spindle-shaped metal particles containing iron as amain component, which can be prevented from undergoing destruction ofparticle shape and deterioration in coercive force, and can show anexcellent dispersibility in vehicle.

[0233] Further, since cobalt is contained in an amount of more than 20atm % and not more than 45 atm % (calculated as Co) based on whole Fe,the oxidation stability of the resultant particles can also be enhanced.Furthermore, since the anti-sintering property is further enhanced asdescribed above, the distribution of particle shape upon variousheat-treatments can be further improved, so that the particle sizedistribution becomes narrower, thereby producing magnetic spindle-shapedmetal particles which are synergistically improved in oxidationstability.

[0234] In the case where the spindle-shaped hematite particles accordingto the present invention is used as starting particles, there can beobtained magnetic spindle-shaped metal particles containing iron as amain component, which can show a high coercive force, a large saturationmagnetization, an excellent oxidation stability and a gooddispersibility in a binder resin. Further, when the magneticspindle-shaped metal particles are used for forming a magnetic coatingfilm, it is possible to produce a magnetic coating film which canexhibit a high coercive force, a good sheet squareness (Br/Bm), a goodsheet SFD and a good weather resistance.

[0235] The reason why the magnetic metal particles having a smallcrystallite size, a high coercive force and a large saturationmagnetization irrespective of the small crystallite size can beproduced, is considered as follows. That is, by adjusting the height ofthe fixed-bed to not more than 30 cm and increasing the temperature inan inert gas atmosphere, the crystallite size of magnetite or wustitecan be increased to such an extent that neither destruction of theparticle shape nor crystal growth in the minor axial direction areinduced due to rapid increase of the water-vapor partial pressure at aninitial stage of the reduction reaction. Further, due to the effects oflowering the specific surface area and decreasing the velocity ofsubsequent reduction reaction into pure iron, the crystal growth ofparticles can be well controlled and the destruction of particle shapecan be considerably suppressed. As a result, it is considered that thecrystallite size can be effectively reduced.

[0236] Further, by adjusting the height of the fixed-bed to not morethan 30 cm, preferably 3 to 30 cm and increasing the temperature in theinert gas atmosphere, the reduction reaction can be inhibited duringsuch a period in which the temperature is elevated with passage of time,till reaching the predetermined reducing temperature, thereby conductingthe reduction reaction under a uniform condition. Further, bychanging-over the inert gas atmosphere to the reducing gas atmosphereand controlling the linear velocity of the reducing gas to 40 to 150cm/s in the specific temperature range, even though the water-vaporpartial pressure is rapidly increased at an initial stage of thereduction reaction, neither destruction of particle shape of magnetiteor wustite nor crystal growth in the minor axial direction are induced,so that the reduction reaction can proceed uniformly over a wholeportion of the fixed-bed. For this reason, it is considered that theobtained magnetic spindle-shaped metal particles can show a highcoercive force and a large saturation magnetization vale as a whole.

[0237] The spindle-shaped goethite particles and the spindle-shapedhematite according to the present invention, are fine particles and freefrom inclusion of dendritic particles, and can exhibit a good particlesize distribution and an appropriate particle shape. Therefore, whenthese particles are used as starting particles, the obtained magneticspindle-shaped metal particles containing iron as a main component notonly are fine particles and free from inclusion of dendritic particlesand can exhibit a good particle size distribution and an appropriateparticle shape, but also can show a high coercive force, an excellentparticle coercive force distribution (SFDr), a large saturationmagnetization, an excellent oxidation stability, a good dispersibilityin a binder resin and a good sheet squareness (Br/Bm) due to the gooddispersibility in a binder resin. Accordingly, the magneticspindle-shaped metal particles containing iron as a main componentaccording to the present invention, can be suitably used as magneticparticles for attaining high recording density, high sensitivity andhigh output.

[0238] Since the spindle-shaped hematite particles according to thepresent invention have a limited content of spinel-type iron oxide andan appropriate crystallite size, the destruction of particle shape uponthe heat-reduction thereof can be prevented as effectively as possible.Accordingly, the spindle-shaped hematite particles according to thepresent invention can be suitably used as starting particles of themagnetic spindle-shaped metal particles containing iron as a maincomponent.

[0239] In the process for producing the magnetic spindle-shaped metalparticles containing iron as a main component according to the presentinvention, the destruction of particle shape upon the heat-reductionstep can be prevented as effectively as possible, and the upper andlower portions of the fixed-bed composed of the starting particles canbe uniformly reduced. As a result, it becomes possible to obtainmagnetic spindle-shaped metal particles satisfying a small crystallitesize, a high coercive force and a large saturation magnetizationsimultaneously.

EXAMPLES

[0240] The present invention will now be described in more detail withreference to the following examples, but the present invention is notrestricted to those examples and various modifications are possiblewithin the scope of the invention.

[0241] (1) The average major axial diameter, the average minor axialdiameter and the aspect ratio of particles were respectively expressedby the average of values measured from electron micrographs.

[0242] (2) The size distribution of the particles is expressed by theratio of a standard deviation to the average major axial diameter.

[0243] The major axial diameters of 300 particles in an electronmicrophotograph (∞200,000 magnification) were measured. The actual majoraxial diameters and the number of the particles were obtained from thecalculation on the basis of the measured values.

[0244] The standard deviation (s) was obtained by the followingequation.

[0245] wherein x₁, x₂, x_(n) represent the determined major axialdiameter of the each specimen, represents an average major axialdiameter determined of the each specimen.

[0246] (3) The specific surface area of particles was expressed by thevalues measured by a BET method using ìMonosorb MS-11î (manufactured byCantachrom Co., Ltd.).

[0247] (4) The X-ray crystallite size (D₀₂₀ and D₁₁₀ of spindle-shapedgoethite particles, D₁₀₄ of spindle-shaped hematite particle or D₁₁₀ ofspindle-shaped magnetic metal particles containing iron as a maincomponent) was expressed by the thickness of crystallite in thedirection perpendicular to the crystal planes (020) and (110) of thespindle-shaped goethite particles, the crystal plane (104) of thespindle-shaped hematite particle or the crystal plane (110) of thespindle-shaped magnetic metal particles containing iron as a maincomponent which were measured by an X-ray diffraction method usingìX-ray diffractometerî (manufactured by Rigaku Denki Kogyo Co., Ltd.)(measuring conditions: target: Fe; tube voltage: 40 kV; and tubecurrent: 40 mA), respectively. The value was calculated from the X-raydiffraction peak curve obtained with respect to the respective crystalplanes by using the following Scherrerís formula:

D ₁₁₀ , D ₀₂₀ or D ₁₀₄ =Kλ/β cos θ

[0248] wherein β is a true half-width of the diffraction peak which wascorrected with respect to the width of machine used (unit: radian); K isa Scherrer constant (0.9); λ is a wavelength of X-ray (Fe Kα-ray 0.1935nm); and θ is a diffraction angle (corresponding to a diffraction peakof the crystal plane (110) and (020) and the crystal plane (104))

[0249] The X-ray diffraction of spindle-shaped hematite particles wasmeasured at a diffraction angle 2θ of 10 to 60_ using the above X-raydiffractometer.

[0250] The smaller the production amount of the spinel-shaped iron oxide(the lower the peak shown with ìBî in the X-ray diffraction peak curve),the lower the σs of the spinel-shaped hematite particles becomes.

[0251] (5) The magnetic properties of magnetic metal particlescontaining iron as a main component and hematite particles, weremeasured using a vibration sample magnetometer ìVSM-3S-15î (manufacturedby Toei Kogyo Co., Ltd.) by applying an external magnetic field of 10kOe.

[0252] (6) The SFDr of particles was measured using a torque/vibrationsample magnetometer (manufactured by Digital Measurement Systems Co.,Ltd.) as follows.

[0253] The magnetic metal particles containing iron as a main componentwere first packed into a capsule. After an external magnetic field of 10kOe was applied to the magnetic metal particles (the direction of theinitially applied external magnetic field was regarded as a positivedirection), the magnetic field was set to zero to measure a residualmagnetization σr (0) of the particles. Thereafter, an external magneticfield of 100 Oe was applied in the reverse direction (negativedirection), and then the magnetic field was set to zero to measure aresidual magnetization σr (100) of the particles. Next, an externalmagnetic field of 10 kOe was applied again in the positive direction,and the magnetic field was set to zero to measure a residualmagnetization σr (0) of the particles. Then, after an external magneticfield of 200 Oe was applied in the negative direction, the magneticfield was set to zero to measure a residual magnetization σr (200) ofthe particles. Subsequently, the measurement of the residualmagnetization was repeated n times in such a manner that the externalmagnetic field applied in the positive direction was kept constant at 10kOe each time, while that applied in the negative direction wasincreased with an increment of 100 Oe, thereby obtaining residualmagnetizations σr (100×n). The measured values of the residualmagnetization σr were plotted based on the values of external magneticfield applied in the negative direction, thereby obtaining a remanence(DC-erased residual magnetization) curve. Using the remanence curve, anexternal magnetic field value Hr at which the residual magnetization waszero was obtained by interpolation method. Further, the half-width valueΔHr of the peak on a differential curve of the above remanence curve wasobtained. The SFDr value of particles was calculated from the followingformula:

SFDr=ΔHr/Hr

[0254] (7) The contents of Co, Al, rare earth elements and other metalelements in the spindle-shaped goethite particles, the spinel-shapedhematite particles or the magnetic spindle-shaped metal particlescontaining iron as a main component, were measured by using aninductively coupled plasma atomic emission spectroscope ìSPS4000î(manufactured by Seiko Denshi Kogyo Co., Ltd.).

[0255] (8) The sheet magnetic characteristics were measured by using asheet test specimen prepared by the following method.

[0256] The respective components as shown below were charged into aplastic bottle, and then mixed and dispersed together for 8 hours usinga paint shaker (manufactured by Reddevil Co., Ltd.), thereby preparing amagnetic coating material. The thus-prepared magnetic coating materialwas applied on a 25 μm-thick polyethylene telephthalate film using anapplicator, and then dried in a magnetic field of 5 kGauss, therebyobtaining the sheet test specimen on which a magnetic coating layerhaving a thickness of 50 μm was formed. Composition of magnetic paint 3mmφ steel balls  800 parts by weight Magnetic spindle-shaped  100 partsby weight metal particles containing iron as a main componentPolyurethane resin containing   20 parts by weight sodium sulfonategroups Cyclohexanone 83.3 parts by weight Methyl ethyl ketone 83.3 partsby weight Toluene 83.3 parts by weight

[0257] (9) The Δσs for evaluating an oxidation stability of thesaturation magnetization σs of particles, and the ΔBm for evaluating anoxidation stability of the sheet saturation magnetic flux density Bm,were measured as follows.

[0258] The test particles or the sheet test specimen were placed in aconstant-temperature oven maintained at 60_C. and a relative humidity of90%, and allowed to stand therein for one week to conduct an accelerateddeterioration test. Thereafter, the test particles and the sheet testspecimen were measured with respect to the saturation magnetization andthe saturation magnetic flux density, respectively. The differences Δσsand ΔBm (as an absolute value), were respectively calculated from thevalues σs and Bm measured before the accelerated test and the values σsíand Bmí measured after the one-week accelerated test.

EXAMPLE 1

[0259] 30 liters of a mixed aqueous alkali solution containing sodiumcarbonate of 25 mol and sodium hydroxide of 20 mol (the concentration ofsodium hydroxide being 28.6 mol % based on mixed alkali) were chargedinto a reaction vessel and the temperature thereof was adjusted to 47_C.while passing a nitrogen gas through the reaction vessel at a linearvelocity of 2.21 cm/s. Then, 20 liters of an aqueous ferrous sulfatesolution containing 20 mol of Fe²⁺ (the concentration of the mixedaqueous alkali solution being 1.75 equivalents based on the ferroussulfate) were charged into the reaction vessel and the contents of thereaction vessel were aged therein for 30 minutes. Thereafter, 4 litersof an aqueous cobalt sulfate solution containing 4.0 mol of Co²⁺(equivalent to 20 atm % (calculated as Co) based on whole Fe) was addedto the reaction vessel and the contents of the reaction vessel were agedtherein for 4 hours and 30 minutes. After aging, air was passed throughthe reaction vessel at a linear velocity of 1.32 cm/s to conduct theoxidation reaction until the percentage of oxidation of Fe²⁺ reached40%, thereby goethite seed crystal particles.

[0260] A part of the water suspension containing the goethite seedcrystal particles wherein the oxidation percentage of Fe²⁺ was proceededto 40%, was taken out and rapidly washed with a diluted aqueous aceticacid solution, followed by filtering and then washing with water. As aresult of the composition analysis of the obtained goethite seed crystalparticles, it was determined that the Fe content was 49.54% by weightand the Co content was 6.43% by weight.

[0261] Next, after the linear velocity of air passing through thereaction vessel was increased to 3.31 cm/s, 1 liter of an aqueousaluminum sulfate solution containing 2.4 mol of Al³⁺ (equivalent to 12atm % (calculated as Al) based on whole Fe) was added into the reactionvessel at a feed rate of not more than 3 ml/sec to conduct the oxidationreaction, and the reaction mixture was washed with water using a filterpress until the electric conductivity reached 60 μS, thereby obtaining apress cake.

[0262] A part of the obtained press cake was dried and pulverized by anordinary method, thereby obtaining goethite particles. As recognizedfrom the transmission electron micrograph shown in FIG. 1, the obtainedgoethite particles were of a spindle shape, and had a BET specificsurface area of 180.3 m²/g, an average major axial diameter of 0.130 μm,a standard deviation σ of 0.0251 μm, a particle size distribution(standard deviation/average major axial diameter) of 0.193, an averageminor axial diameter of 0.0173 μm, an aspect ratio (average major axialdiameter/average minor axial diameter) of 7.5:1, an X-ray crystallite(D₀₂₀) of 22.4 nm, an X-ray crystallite (D₁₁₀) of 8.9 nm, and an X-raycrystallite size ratio (D₀₂₀/D₁₁₀) of 2.52.

[0263] Further, the obtained goethite particles contained no dendriticparticles. The obtained goethite particles comprised 44.5% by weight ofFe based on the weight of the particle, 9.39% by weight of Co based onthe weight of the particle and 2.58% by weight of Al based on the weightof the particle. As a result of the comparison of these values withthose of the goethite seed crystal particles, it was determined that theCo content in the seed portion was 12.3 atm % (calculated as Co) basedon the Fe in the seed portion; the relationship of the Co concentrationof the seed portion of the goethite particle with that of the goethiteparticle was 61.5:100 and the relationship of the Co concentration ofthe surface layer portion of the goethite particle with that of thegoethite particle was 125.7:100, when the Co concentration of thegoethite particle is 100; the Co content in a whole particle was 20 atm% (calculated as Co) based on whole Fe; and the Al content was 12 atm %(calculated as Al) based on whole Fe. Further, it was determined that Alexisted only in the surface layer portion.

EXAMPLE 2

[0264] A press cake containing 1,000 g of the spindle-shaped goethiteparticles obtained in Example 1 (7.97 mol as Fe) was sufficientlydispersed in 40 liters of water. 2 liters of an aqueous yttrium nitratesolution containing 245 g of yttrium nitrate hexahydrate (equivalent to8 atm % (calculated as Y) based on whole Fe) was added to thedispersion, and then stirred. Further, after a 25.0 wt % aqueous sodiumcarbonate solution as a precipitating agent was added so as to adjustthe pH value of the dispersion to 9.5, the dispersion was washed withwater using a filter press. The obtained press cake was extrusion-moldedusing a compression molding machine equipped with a mold plate having anorifice diameter of 3 mm, and the resultant molded product were dried at120_C., thereby obtaining goethite particles coated with the yttriumcompound. The Co content in the obtained goethite particles was 20 atm %(calculated as Co) based on whole Fe; the Al content thereof was 12 atm% (calculated as Al) based on whole Fe; and the Y content thereof was 8atm % (calculated as Y) based on whole Fe. Further, it was determinedthat Al existed only in the intermediate layer portion and yttriumexisted only in the outer layer portion.

[0265] The spindle-shaped goethite particles coated with the yttriumcompound were heat-dehydrated in air at 600_C. for 1 hours, therebyproducing spindle-shaped hematite particles having an outer layercomposed of the yttrium compound.

[0266] As recognized from the transmission electron micrograph shown inFIG. 2, the obtained spindle-shaped hematite particles had an averagemajor axial diameter of 0.121 μm, a standard deviation σ of 0.0223 μm, aparticle size distribution (standard deviation/average major axialdiameter) of 0.184, an average minor axial diameter of 0.0166 μm, anaspect ratio (average major axial diameter/average minor axial diameter)of 7.3:1 and a BET specific surface area of 87.3 m²/g. The Co content inthe particles was 20 atm % (calculated as Co) based on whole Fe; the Alcontent was 12 atm % (calculated as Al) based on whole Fe; and the Ycontent was 8 atm % (calculated as Y) based on whole Fe.

EXAMPLE 3

[0267] 100 g of the spindle-shaped hematite particles having the outerlayer composed of the yttrium compound which were obtained in Example 2,were charged into a fixed-bed reducing apparatus having an innerdiameter of 72 mm. While a hydrogen (H₂) gas was passed through thereducing apparatus at a flow rate of 35 liter/min, the spindle-shapedhematite particles were heat-reduced at 600_C. for 2 hours. After thehydrogen gas was replaced with a nitrogen gas, the particles were cooledto 80_C., and then the oxygen partial pressure in the reducing apparatuswas gradually increased by passing a water vapor therethrough until theoxygen content therein reached the same content as in air, therebyforming a stable oxide layer on the surface of each particle.

[0268] As recognized from the transmission electron micrograph shown inFIG. 3, the obtained magnetic metal particles containing iron as a maincomponent and further containing Co, Al and Y, had an average majoraxial diameter of 0.108 μm, a standard deviation a of 0.0171 μm, aparticle size distribution (standard deviation/average major axialdiameter) of 0.158, an average minor axial diameter of 0.0158 μm, anaspect ratio (average major axial diameter/average minor axial diameter)of 6.8:1, a BET specific surface area of 47.0 m²/g and an X-raycrystallite (D₁₁₀) of 15.2 nm. Further, the magnetic metal particlescontaining iron as a main component had a spindle shape and a uniformparticle size, and contained no dendritic particles. The Co content inthe particles was 20 atm % (calculated as Co) based on whole Fe; the Alcontent was 12 atm % (calculated as Al) based on whole Fe; and the Ycontent was 8 atm % (calculated as Y) based on whole Fe. As to themagnetic properties of the magnetic metal particles, the coercive forcethereof was as high as 2,310 Oe; the saturation magnetization σs was141.0 emu/g; the squareness (σr/σs) was 0.535; the particle SFDr was0.710; and the oxidation stability Δσs of the saturation magnetizationwas 8.6% as an absolute value (measured value: −8.6%). Further, as tosheet magnetic characteristics, the sheet coercive force Hc was 2,365Oe; the sheet squareness (Br/Bm, wherein Bm was 3,901G), was 0.870; thesheet SFD was 0.395; and ΔBm was 6.0% (measured value: −6.0%).

EXAMPLE 4

[0269] <Production Spindle-Shaped Goethite Particles>

[0270] 30 liters of a mixed aqueous alkali solution containing sodiumcarbonate of 25 mol and sodium hydroxide of 20 mol (the concentration ofsodium hydroxide being equivalent to 28.6 mol % based on mixed alkali)were charged into a reaction vessel and the temperature thereof wasadjusted to 47_C. while passing a nitrogen gas through the reactionvessel at a linear velocity of 2.21 cm/s. Then, 20 liters of an aqueousferrous sulfate solution containing 20 mol of Fe²⁺ (the concentration ofthe mixed aqueous alkali solution being 1.75 equivalents based on theferrous sulfate) were charged into the reaction vessel and the contentsof the reaction vessel were aged therein for 20 minutes. Thereafter, 4liters of an aqueous cobalt sulfate solution containing 4.2 mol of Co²⁺(equivalent to 21 atm % (calculated as Co) based on whole Fe) was addedto the reaction vessel and the contents of the reaction vessel werefurther aged therein for 4 hours and 40 minutes. After aging, air waspassed through the reaction vessel at a linear velocity of 1.32 cm/s toconduct the oxidation reaction until the percentage of oxidation of Fe²⁺reached 40%, thereby goethite seed crystal particles.

[0271] A part of the water suspension containing the goethite seedcrystal particles wherein the oxidation percentage of Fe²⁺ was proceededto 40%, was taken out and rapidly washed with a diluted aqueous aceticacid solution, followed by filtering and then washing with water. As aresult of the composition analysis of the obtained goethite seed crystalparticles, it was determined that the Fe content was 48.60% by weightand the Co content was 6.66% by weight.

[0272] Next, after the linear velocity of air passing through thereaction vessel was increased to 3.31 cm/s, 1 liter of an aqueousaluminum sulfate solution containing 2.4 mol of Al³⁺ (equivalent to 12atm % (calculated as Al) based on whole Fe) was added into the reactionvessel at a feed rate of not more than 3 ml/sec to conduct the oxidationreaction, and the reaction mixture was washed with water using a filterpress until the electric conductivity reached 60 μS, thereby obtaining apress cake.

[0273] A part of the obtained press cake was dried and pulverized by anordinary method, thereby obtaining goethite particles. As recognizedfrom the transmission electron micrograph shown in FIG. 9, the obtainedgoethite particles were of a spindle shape, and had a BET specificsurface area of 179.2 m²/g, an average major axial diameter of 0.131 μm,a standard deviation σ of 0.0250 μm, a particle size distribution(standard deviation/average major axial diameter) of 0.191, an averageminor axial diameter of 0.0175 μm and an aspect ratio (average majoraxial diameter/average minor axial diameter) of 7.5:1. Further, theobtained goethite particles contained no dendritic particles. Inaddition, the obtained goethite particles comprised 44.0% by weight ofFe based on the weight of the particle, 9.76% by weight of Co based onthe weight of the particle and 2.55% by weight of Al based on the weightof the particle. As a result of the comparison of these values withthose of the goethite seed crystal particles, it was determined that theCo content in the seed portion was 13.0 atm % (calculated as Co) basedon the Fe in the seed portion; the relationship of the Co concentrationof the seed portion of the goethite particle with that of the goethiteparticle was 61.9:100 and the relationship of the Co concentration ofthe surface layer portion of the goethite particle with that of thegoethite particle was 125.4:100, when the Co concentration of thegoethite particle is 100; the relationship of the Co concentration ofthe seed portion of the goethite particle with that of the goethiteparticle was 61.5:100 and the relationship of the Co concentration ofthe surface layer portion of the goethite particle with that of thegoethite particle was 125.7:100, when the Co concentration of thegoethite particle is 100; and the Al content was 12 atm % (calculated asAl) based on whole Fe. Further, it was determined that Al existed onlyin the surface layer portion.

EXAMPLE 5

[0274] The press cake containing 1,000 g of the spindle-shaped goethiteparticles obtained in Example 4 (7.88 mol as Fe) was sufficientlydispersed in 40 liters of water. 2 liters of an aqueous yttrium nitratesolution containing 243 g of yttrium nitrate hexahydrate (equivalent to8 atm % (calculated as Y) based on whole Fe in the spindle-shapedgoethite particles) and 2 liters of an aqueous cobalt sulfate solutioncontaining 197 g of cobalt sulfate heptahydrate (equivalent to 9 atm %(calculated as Co) based on whole Fe in the spindle-shaped goethiteparticles) were added to the dispersion, and then stirred. Further,after a 25.0 wt % aqueous sodium carbonate solution as a precipitatingagent was added so as to adjust the pH value of the dispersion to 9.5,the dispersion was washed with water using a filter press. The obtainedpress cake was extrusion-molded using a compression molding machineequipped with a mold plate having an orifice diameter of 3 mm, and theresultant molded particles were dried at 120_C., thereby obtainingspindle-shaped goethite particles coated with the yttrium and cobaltcompounds. The Co content in the obtained spindle-shaped goethiteparticles was 30 atm % (calculated as Co) based on whole Fe; the Alcontent was 12 atm % (calculated as Al) based on whole Fe; and the Ycontent was 8 atm % (calculated as Y) based on whole Fe. Further, it wasdetermined that Al existed only in the intermediate layer portion andyttrium existed only in the outer layer portion.

[0275] The spindle-shaped goethite particles coated with the yttrium andcobalt compounds were heat-dehydrated in air at 300_C., and furtherheat-dehydrated in the same atmosphere at 600_C. for 1 hours, therebyproducing spindle-shaped hematite particles.

[0276] As recognized from the transmission electron micrograph shown inFIG. 10, the obtained spindle-shaped hematite particles had an averagemajor axial diameter of 0.122 μm, a standard deviation σ of 0.0218 μm, aparticle size distribution (standard deviation/average major axialdiameter) of 0.179, an average minor axial diameter of 0.0168 μm, anaspect ratio (average major axial diameter/average minor axial diameter)of 7.3:1 and a BET specific surface area of 95.7 m²/g. The Co content inthe spindle-shaped hematite particles was 30 atm % (calculated as Co)based on whole Fe; the Al content thereof was 12 atm % (calculated asAl) based on whole Fe; and the Y content thereof was 8 atm % (calculatedas Y) based on whole Fe. In addition, the crystallite size (D₁₀₄) of thespindle-shaped hematite particles was 76_ and the saturationmagnetization thereof was 1.0 emu/g.

EXAMPLE 6

[0277] <Production of Magnetic Spindle-Shaped Metal Particles>

[0278] 100 g of the spindle-shaped hematite particles obtained inExample 5, were charged into a fixed-bed reducing apparatus having aninner diameter of 72 mm. While a hydrogen (H₂) gas was passed throughthe reducing apparatus at a flow rate of 35 liter/min, thespindle-shaped hematite particles were heat-reduced at 600_C. for 2hours. The hydrogen gas was replaced with a nitrogen gas, and thetemperature of the reducing apparatus was cooled to 80_C. Then, theoxygen partial pressure in the reducing apparatus was graduallyincreased while passing steam therethrough until the oxygen contenttherein reached the same content as in air, thereby forming a stableoxide layer on the surface of each particle.

[0279] As recognized from the transmission electron micrograph shown inFIG. 11, the obtained magnetic spindle-shaped metal particles had anaverage major axial diameter of 0.105 μm, a standard deviation σ of0.0163 μm, a particle size distribution (standard deviation/averagemajor axial diameter) of 0.155, an average minor axial diameter of0.0154 μm, an aspect ratio (average major axial diameter/average minoraxial diameter) of 6.8:1, a BET specific surface area of 50.1 m²/g andan X-ray crystallite size (D₁₁₀) of 15.2 nm. Further, the magnetic metalparticles had a spindle shape and a uniform particle size, and containedno dendritic particles. The Co content in the particles was 30 atm %(calculated as Co) based on whole Fe; the Al content was 12 atm %(calculated as Al) based on whole Fe; and the Y content was 8 atm %(calculated as Y) based on whole Fe. As to the magnetic properties ofthe magnetic spindle-shaped metal particles containing iron as a maincomponent, the coercive force thereof was as high as 2,362 Oe; thesaturation magnetization σs was 140.5 emu/g; the squareness (σr/σs) was0.543; and the oxidation stability Δσs of the saturation magnetizationwas 7.0% as an absolute value (measured value: −7.0%). Further, as tomagnetic characteristics of magnetic coating film, the film coerciveforce Hc was 2,411 Oe; the squareness (Br/Bm) was 0.873; the SFD was0.382; and ΔBm was 5.1% (measured value: −5.1%).

EXAMPLE 7

[0280] <Production of Magnetic Spindle-Shaped Metal Particles>

[0281] 500 g of molded granules (average particle size: 2.6 mm) composedof the spindle-shaped hematite particles obtained in Example 6 werecharged into a fixed-bed reducing apparatus having an inner diameter of72 mm and a bed height of 27 cm, and heated up to 600_C. while passing anitrogen gas through the reducing apparatus at a linear velocity of 20cm/s. After the nitrogen gas was replaced with a hydrogen gas, thecontents of the reducing gas was heat-reduced at 600_C. while passingthe hydrogen gas at a linear velocity of 50 cm/s until the dew point ofthe exhaust gas reached −30_C. Thereafter, the hydrogen gas was replacedagain with the nitrogen gas, and the reducing apparatus was cooled downto 80_C. Steam and then air were mixed in the nitrogen gas, and themixed gas was passed through the reducing apparatus to graduallyincrease the oxygen partial pressure, thereby forming a stable oxidelayer on each particle. As a result, there was obtained magneticspindle-shaped metal particles.

[0282] A part (about 10 g) of the thus obtained magnetic spindle-shapedmetal particles were taken out from both a lower bed portion (bedheight: not more than 3 cm) and an upper bed portion (bed height: notless than 25 cm), and measured as to magnetic properties and crystallitesize thereof.

[0283] The obtained magnetic metal particles had an average major axialdiameter of 0.104 μm, a standard deviation σ of 0.0165 μm, a particlesize distribution (standard deviation/average major axial diameter) of0.159, an average minor axial diameter of 0.0153 μm, an average aspectratio (average major axial diameter/average minor axial diameter) of6.8:1, a BET specific surface area of 52.4 m²/g and a crystallite size(D₁₁₀) of 153_. Further, the magnetic metal particles had a spindleshape and a uniform particle size, and contained no dendritic particles.The Co content in the particles was 30 atm % (calculated as Co) based onwhole Fe; the Al content was 12 (calculated as Al) atm % based on wholeFe; and the Y content was 8 atm % (calculated as Y) based on whole Fe.

[0284] As to the magnetic properties of the magnetic spindle-shapedmetal particles, the coercive force Hc thereof was 2,321 Oe; thesaturation magnetization σs was 146.5 emu/g; the squareness (σr/σs) was0.540; and the oxidation stability Δσs of the saturation magnetizationwas 9.3% as an absolute value (measured value: −9.3%). Further, as tomagnetic characteristics of magnetic coating film, the coercive forcewas 2,370 Oe; the squareness (Br/Bm) was 0.875; the SFD was 0.380; andthe oxidation stability ΔBm was 7.1% as an absolute value (measuredvalue: −7.1%).

[0285] Incidentally, the magnetic spindle-shaped metal particles takenout from the lower bed portion, showed a coercive force Hc of 2,335 Oe,a saturation magnetization σs of 145.9 emu/g, a squareness (σr/σs) of0.541 and a crystallite size (D₁₁₀) of 152_. Whereas, the magneticspindle-shaped metal particles taken out from the upper bed portionshowed a coercive force Hc of 2,311 Oe, a saturation magnetization σs of146.8 emu/g, a squareness (σr/σs) of 0.538 and a crystallite size (D₁₁₀)of 155_.

EXAMPLES 8 TO 14

[0286] The same procedure as defined in Example 1 was conducted exceptthat production conditions of the spindle-shaped goethite particles,i.e., the production reaction conditions of goethite seed crystalparticles and the growth reaction conditions thereof, were varied asshown in Table 1, thereby obtaining spindle-shaped goethite particles.Various properties of the obtained spindle-shaped goethite particles areshown in Table 2.

[0287] Further, in FIG. 4, there is shown an electron micrograph of aparticle structure of the goethite particles obtained in Example 9.

COMPARATIVE EXAMPLE 1

[0288] The same procedure as defined in Example 1 was conducted exceptthat the production reaction conditions of goethite particles werechanged such that the linear velocity of the oxygen-containing gaspassed through the reaction vessel upon the growth reaction was adjustedto 1.32 cm/s equal to that upon the production reaction of goethite seedcrystal particles, thereby producing goethite particles.

[0289] As recognized from the transmission electron micrograph shown inFIG. 7, the obtained goethite particles showed crystal growth in theminor axial direction, resulting in deterioration in aspect ratio andparticle size distribution thereof.

COMPARATIVE EXAMPLE 2

[0290] The same procedure as defined in Example 1 was conducted exceptthat the production reaction conditions of goethite particles werechanged such that the linear velocity of the oxygen-containing gaspassed through the reaction vessel upon the growth reaction was adjustedto 1.98 cm/s which was 1.5 times that upon the production reaction ofgoethite seed crystal particles, thereby producing goethite particles.

[0291] The obtained goethite particles showed crystal growth in theminor axial direction, resulting in deterioration in aspect ratio andparticle size distribution thereof.

COMPARATIVE EXAMPLE 3

[0292] The same procedure as defined in Example 1 was conducted exceptthat the production reaction conditions of goethite particles werechanged such that Al to be added, was added when the percentage ofoxidation of Fe²⁺ reached 100%, i.e., when no unreacted Fe²⁺ remainedtherein, thereby producing goethite particles.

[0293] The obtained goethite particles showed crystal growth in themajor axial direction, thereby improving an aspect ratio thereof, butthe particle size distribution thereof was deteriorated.

COMPARATIVE EXAMPLE 4

[0294] The same procedure as defined in Example 1 was conducted exceptthat the contents of the Co and Al compounds added were changed to 5 atm% (calculated as Co) and 3 atm % (calculated as Al) based on Fe, andother conditions were shown in Table 1, thereby producing goethiteparticles.

[0295] The obtained goethite particles showed crystal growth in themajor axial direction, thereby improving an aspect ratio thereof, butthe particle size distribution thereof was deteriorated.

EXAMPLES 15 TO 20 AND COMPARATIVE EXAMPLES 5 TO 9

[0296] <Production of Spindle-Shaped Hematite Particles>

[0297] The same procedure as defined in Example 2 was conducted exceptthat kind of the spindle-shaped goethite particles as precursor, kindand amount of the coating material used for the anti-sinteringtreatment, the heat-dehydration temperature and the subsequentheat-treatment temperature were varied, thereby producing spindle-shapedhematite particles. Production conditions and various properties of theobtained spindle-shaped hematite particles are shown in Table 3.

[0298]FIG. 5 is a transmission electron micrograph showing a particlestructure of the hematite particles obtained in Example 16.

EXAMPLES 21 TO 26 AND COMPARATIVE EXAMPLES 10 TO 14

[0299] <Production of Magnetic Spindle-Shaped Metal Particles ContainingIron As A Main Component>

[0300] The same procedure as defined in Example 3 was conducted exceptthat kind of particles to be treated, kind and amount of the coatingmaterial used for the anti-sintering treatment, the heating temperatureand the reducing temperature upon the heat-reduction step were varied,thereby producing magnetic metal particles containing iron as a maincomponent. Reduction conditions and various properties of the obtainedmagnetic metal particles containing iron as a main component, are shownin Table 4.

[0301]FIG. 6 is a transmission electron micrograph showing a particlestructure of the magnetic metal particles containing iron as a maincomponent which were obtained in Example 22. Further, FIG. 8 is atransmission electron micrograph showing a particle structure of themagnetic metal particles containing iron as a main component which wereobtained in Comparative Example 10.

EXAMPLE 27

[0302] The spindle-shaped goethite particles were subjected toanti-sintering treatment, and then directly heat-reduced in hydrogen at600_C., thereby producing magnetic spindle-shaped metal particlescontaining iron as a main component. Production conditions and variousproperties of the obtained magnetic metal particles containing iron as amain component, are shown in Table 4.

GOETHITE PARTICLES 1 TO 4

[0303] Four kinds of spindle-shaped goethite particles as startingparticles having various properties shown in Table 5, were prepared inthe similar manner to production method as defined in Example 4.

EXAMPLES 28 TO 31 AND COMPARATIVE EXAMPLES 15 TO 18

[0304] The same procedure as defined in Example 5 was conducted exceptthat kind of starting particles, kind and composition ratio of thecoating material used for the anti-sintering treatment, and thedehydrating temperature, the heating temperature and the atmosphere upontransforming the starting particles into hematite particles, werevaried, thereby producing spindle-shaped hematite particles.Anti-sintering treatment conditions and conditions of the production ofhematite particles are shown in Table 6, and various properties of theobtained spindle-shaped hematite particles are shown in Table 7.

REFERENCE EXAMPLE 1

[0305] According to the method of transforming goethite into hematite asdescribed in Japanese Patent Application Laid-Open (KOKAI) No. 9-316461(1997), spindle-shaped goethite particles which were previouslysubjected to an anti-sintering treatment, were heated at 600_C., therebyproducing spindle-shaped hematite particles.

REFERENCE EXAMPLE 2

[0306] According to the method of transforming goethite into hematite asdescribed in Japanese Patent Application Laid-Open (KOKAI) No. 9-316461(1997), spindle-shaped goethite particles which were previouslysubjected to anti-sintering treatment, were heated at 650_C., therebyproducing spindle-shaped hematite particles.

[0307] The treatment conditions are shown in Table 6, and variousproperties of the obtained spindle-shaped hematite particles are shownin Table 7.

EXAMPLES 31 TO 35, REFERENCE EXAMPLES 3 TO 4 AND COMPARATIVE EXAMPLES 19TO 22

[0308] Respective spindle-shaped hematite particles as shown in Table 7,were heat-reduced at 600_C. in the same manner as in Example 6, therebyproducing magnetic spindle-shaped metal particles.

[0309] Various properties of the obtained magnetic spindle-shaped metalparticles are shown in Table 8.

[0310] The respective magnetic spindle-shaped metal particles as shownin Table 8, were treated in the same manner as in Example 6, therebyproducing magnetic coating films.

[0311] Various properties of the obtained magnetic coating films areshown in Table 9.

Spindle-Shaped Goethite Particles 5 to 6

[0312] Two kinds of spindle-shaped goethite particles as startingparticles having various properties shown in Table 10, were prepared inthe similar manner to production method as defined in Example 4.

Spindle-Shaped Hematite Particles 1 to 4

[0313] The same procedure as defined in Example 5 was conducted exceptthat kind of the spindle-shaped goethite particles, kind and amount ofthe coating material used for the anti-sintering treatment and theheat-dehydrating temperature were varied, thereby producingspindle-shaped hematite particles. Production conditions and variousproperties of the obtained spindle-shaped hematite particles are shownin Tables 11 and 12.

EXAMPLES 36 TO 40 AND COMPARATIVE EXAMPLES 23 TO 29

[0314] The same procedure as defined in Example 7 was conducted exceptthat kind of the spindle-shaped hematite particles, the fixed-bedheight, kind of the heating gas, kind of the reducing gas, the linearvelocity and the reducing temperature were varied, thereby producingmagnetic spindle-shaped metal particles. Production conditions andvarious properties of the obtained magnetic spindle-shaped metalparticles are shown in Tables 13 and 14.

[0315] In Example 36 and Comparative Example 23, a part of the magneticspindle-shaped metal particles were taken out from the lower and upperportions of the fixed-bed in the same manner as in Example 7. Variousproperties of the magnetic spindle-shaped metal particles taken out fromthe fixed-bed, are shown in Table 15.

[0316] Using the respective magnetic spindle-shaped metal particles asshown in Table 14, magnetic coating films were produced in the samemanner as in Example 7. Various properties of the obtained magneticcoating films are shown in Table 16. TABLE 1 Production ofspindle-shaped goethite particles Production reaction of spindle-shapedgoethite seed crystal particles Mixed aqueous alkali solution Alkaliratio: Aqueous alkali Aqueous alkali 1/2 ∞ Examples carbonate hydroxidealkali and solution solution hydroxide/ Comparative Amount Amount wholealkali Examples Kind used (mol) Kind used (mol) (%) Example 8 Na₂CO₃ 25NaOH 20 28.6 Example 9 Na₂CO₃ 25 NaOH 20 28.6 Example 10 Na₂CO₃ 25 NaOH20 28.6 Example 11 Na₂CO₃ 25 NaOH 24 32.4 Example 12 Na₂CO₃ 25 NaOH 2432.4 Example 13 Na₂CO₃ 25 NaOH 24 32.4 Example 14 Na₂CO₃ 25 NaOH 20 28.6Comparative Na₂CO₃ 25 NaOH 20 28.6 Example 1 Comparative Na₂CO₃ 25 NaOH20 28.6 Example 2 Comparative Na₂CO₃ 25 NaOH 20 28.6 Example 3Comparative Na₂CO₃ 25 NaOH 15 23.1 Example 4 Production ofspindle-shaped goethite particles Production reaction of spindle-shapedgoethite seed crystal particles Aging Aqueous ferrous Equivalent LinearExamples salt solution ratio: velocity of and Amount whole Tem- nitrogenComparative used alkali/ perature Time passed Examples Kind (mol)Fe²⁺(*) (_C.) (hr) (cm/s) Example 8 FeSO₄ 20 1.75 47 5 3.31 Example 9FeSO₄ 20 1.75 47 5 2.21 Example 10 FeSO₄ 20 1.75 47 5 2.21 Example 11FeSO₄ 20 1.85 47 5 4.42 Example 12 FeSO₄ 20 1.85 47 5 4.42 Example 13FeSO₄ 20 1.85 47 5 4.42 Example 14 FeSO₄ 20 1.75 47 5 2.21 ComparativeFeSO₄ 20 1.75 47 5 2.21 Example 1 Comparative FeSO₄ 20 1.75 47 5 2.21Example 2 Comparative FeSO₄ 20 1.75 47 5 2.21 Example 3 ComparativeFeSO₄ 20 1.625 47 5 2.21 Example 4 Production of spindle-shaped goethiteparticles Production reaction of spindle-shaped goethite seed crystalparticles Examples Cobalt compound Linear and Amount Timing velocity ofTem- Comparative used of air passed perature Examples Kind (mol)addition (cm/s) (_C.) Example 8 CoSO₄ 4  0.5 h 1.99 47 after agingExample 9 CoSO₄ 2  2.0 h 1.99 47 after aging Example 10 CoSO₄ 2  2.0 h1.55 47 after aging Example 11 CoSO₄ 6 0.25 h 1.10 47 after agingExample 12 CoSO₄ 6 0.25 h 1.10 47 after aging Example 13 CoSO₄ 6 0.25 h1.10 47 after aging Example 14 CoSO₄ 4  0.5 h 1.32 47 after agingComparative CoSO₄ 4  0.5 h 1.32 47 Example 1 after aging ComparativeCoSO₄ 4  0.5 h 1.32 47 Example 2 after aging Comparative CoSO₄ 4  0.5 h1.32 47 Example 3 after aging Comparative CoSO₄ 1 4.75 h 2.65 47 Example4 after aging Production of spindle-shaped goethite particles Growthreaction of seed crystal particles Aluminum compound Amount of airpassed Timing of Ratio addition to linear Examples (percentage velocityand Amount of Linear upon pro- Comparative used oxidation of velocityduction of Examples Kind (mol) Fe²⁺: %)(*) (cm/s) seed crystal Example 8Aluminum 1.6 60 4.42 2.22 sulfate Example 9 Aluminum 2.0 50 4.42 2.22sulfate Example 10 Aluminum 1.4 40 3.31 2.14 sulfate Example 11 Aluminum2.4 50 3.31 3 sulfate Example 12 Aluminum 1.4 50 3.31 3 sulfate Example13 Aluminum 2.0 70 3.31 3 sulfate Example 14 Aluminum 2.4 40 3.31 2.5sulfate Comparative Aluminum 2.4 40 1.32 1 Example 1 sulfate ComparativeAluminum 2.4 40 1.98 1.5 Example 2 sulfate Comparative Aluminum 2.4 1003.31 2.5 Example 3 sulfate Comparative Aluminum 0.6 70 2.65 1 Example 4sulfate

[0317] TABLE 2 Properties of goethite particles Average Examples majorand axial Standard Comparative diameter: devia- Examples Kind Shape 1(μm) tion: σ Example 8 Goethite Spindle- 0.122 0.0241 particles shapedExample 9 Goethite Spindle- 0.132 0.0262 particles shaped Example 10Goethite Spindle- 0.135 0.0262 particles shaped Example 11 GoethiteSpindle- 0.121 0.0238 particles shaped Example 12 Goethite Spindle-0.129 0.0249 particles shaped Example 13 Goethite Spindle- 0.142 0.0278particles shaped Example 14 Goethite Spindle- 0.130 0.0251 particlesshaped Comparative Goethite Spindle- 0.126 0.0311 Example 1 particlesshaped Comparative Goethite Spindle- 0.127 0.0316 Example 2 particlesshaped Comparative Goethite Spindle- 0.146 0.0389 Example 3 particlesshaped Comparative Goethite Spindle- 0.131 0.0326 Example 4 particlesshaped Properties of goethite particles particle Average BET Examplessize minor specific and distri- axial surface Comparative bution:diameter Aspect area Examples σ/1 (μm) ratio (m²/g) Example 8 0.1980.0165 7.4 183.0 Example 9 0.196 0.0181 7.3 175.9 Example 10 0.1940.0175 7.7 166.5 Example 11 0.197 0.0170 7.1 207.6 Example 12 0.1930.0179 7.2 192.5 Example 13 0.196 0.0195 7.3 184.3 Example 14 0.1930.0173 7.5 180.3 Comparative 0.247 0.0198 6.4 143.9 Example 1Comparative 0.249 0.0195 6.5 160.5 Example 2 Comparative 0.266 0.01828.0 201.3 Example 3 Comparative 0.249 0.0168 7.8 131.4 Example 4Properties of goethite particles Composition ratio of seed portion Cocontent Relationship in seed of Co concentra- crystal tion in seedExamples to Fe in crystal to Co Al and seed Co concen- content: content:Comparative crystal tration in Co/Fe Al/Fe Examples (atm %) particle(atm %) (atm %) Example 8 16.3 81.5 20 8 Example 9 7.3 72.5 10 10Example 10 6.2 61.5 10 7 Example 11 21.8 72.5 30 12 Example 12 21.7 72.330 7 Example 13 26.6 88.8 30 10 Example 14 12.3 61.5 20 12 Comparative12.3 61.5 20 12 Example 1 Comparative 12.3 61.5 20 12 Example 2Comparative 20.0 100 20 12 Example 3 Comparative 4.4 88.8 5 3 Example 4

[0318] TABLE 3 Production conditions of hematite particlesAnti-sintering agent Compound of rare Heat-treatment Examples earthelement Other compound Heating and Com- Amount Amount tem- parativeadded added perature Atmos- Examples Kind (g) Kind (g) (_C.) phereExample Yttrium 245 — — 600 Air 15 nitrate Example Neodymium 225 — — 700Air 16 nitrate Example Neodymium 150 — — 700 Air 17 nitrate ExampleYttrium 230 — — 600 Air 18 nitrate Example Yttrium 340 — — 600 Air 19nitrate Example Praseo- 260 — — 600 Air 20 dymium nitrate Com- Yttrium245 — — 600 Air parative nitrate Example 5 Com- Yttrium 245 — — 600 Airparative nitrate Example 6 Com- Yttrium 245 — — 600 Air parative nitrateExample 7 Com- — 150 Boric 150 400 Air parative acid Example 8 Com-Yttrium 605 — — 600 Air parative nitrate Example 9 Production conditionsof hematite particles Properties of hematite particles Examples andAverage major Standard Comparative axial diameter: deviation: ExamplesKind Shape 1 (μm) σ Example 15 Hematite Spindle- 0.110 0.0201 particlesshaped Example 16 Hematite Spindle- 0.123 0.0230 particles shapedExample 17 Hematite Spindle- 0.126 0.0236 particles shaped Example 18Hematite Spindle- 0.111 0.0206 particles shaped Example 19 HematiteSpindle- 0.120 0.0222 particles shaped Example 20 Hematite Spindle-0.131 0.0247 particles shaped Comparative Hematite Spindle- 0.118 0.0290Example 5 particles shaped Comparative Hematite Spindle- 0.119 0.0291Example 6 particles shaped Comparative Hematite Spindle- 0.136 0.0342Example 7 particles shaped Comparative Hematite Spindle- 0.121 0.0279Example 8 particles shaped Comparative Hematite Spindle- 0.118 0.0242Example 9 particles shaped Production conditions of hematite particlesProperties of hematite particles Examples particle Average BET and sizeminor axial specific Comparative distribution: diameter Aspect surfacearea Examples σ/1 (μm) ratio (m²/g) Example 15 0.183 0.0156 7.0 85.1Example 16 0.187 0.0173 7.1 63.6 Example 17 0.188 0.0168 7.5 62.3Example 18 0.185 0.0162 6.9 98.5 Example 19 0.185 0.0170 7.1 92.3Example 20 0.189 0.0189 6.9 94.8 Comparative 0.246 0.0190 6.2 64.3Example 5 Comparative 0.245 0.0187 6.4 72.0 Example 6 Comparative 0.2510.0175 7.8 82.1 Example 7 Comparative 0.230 0.0160 7.6 69.8 Example 8Comparative 0.205 0.0160 7.4 110.4 Example 9 Production conditions ofhematite particle Properties of hematite particles Composition ratio ofseed portion Relationship Co of Co con- Content content in centration inof rare Examples seed crystal seed crystal Co Al earth and to Fe in toCo con- content: content: Element: Comparative seed crystal centrationCo/Fe Al/Fe Re/Fe Examples (atm %) in particle (atm %) (atm %) (atm %)Example 15 16.3 81.5 20 8 Ln: 8 Example 16 7.3 72.5 10 10 Ln: 6 Example17 6.2 61.5 10 7 Ln: 4 Example 18 21.8 72.5 30 12 Ln: 8 Example 19 21.772.3 30 7 Ln: 12 Example 20 26.6 88.8 30 10 Ln: 8 Comparative 12.3 61.520 12 Ln: 8 Example 5 Comparative 12.3 61.5 20 12 Ln: 8 Example 6Comparative 20.0 100 20 12 Ln: 8 Example 7 Comparative 4.4 88.8 5 3 B:10 Example 8 Comparative 12.3 61.4 20 12 Ln: 20 Example 9

[0319] TABLE 4 Properties of magnetic Heat- metal particles containingExamples reduction iron as main component* and Reducing Average majorComparative temperature axial diameter: Standard Examples (_C.) 1 (μm)deviation: σ Example 21 600 0.102 0.0161 Example 22 500 0.110 0.0174Example 23 500 0.114 0.0183 Example 24 600 0.100 0.0159 Example 25 6000.106 0.0168 Example 26 600 0.118 0.0189 Example 27 note 3) 0.105 0.0192600 Comparative 600 0.105 0.0220 Example 10 Comparative 600 0.108 0.0225Example 11 Comparative 600 0.123 0.0269 Example 12 Comparative 400 0.1090.0229 Example 13 Comparative 600 0.110 0.0275 Example 14 Properties ofmagnetic metal particles Examples containing iron as main component andparticle size Average minor BET specific Comparative distribution: axialdiameter Aspect surface area Examples σ/1 (μm) ratio (m²/g) Example 210.157 0.0152 6.7 46.6 Example 22 0.159 0.0165 6.6 48.7 Example 23 0.1600.0163 7.0 45.0 Example 24 0.158 0.0155 6.5 48.7 Example 25 0.159 0.01616.6 49.8 Example 26 0.160 0.0182 6.5 48.1 Example 27 0.183 0.0162 6.542.3 Comparative 0.210 0.0181 5.8 43.8 Example 10 Comparative 0.2080.0183 5.9 44.9 Example 11 Comparative 0.219 0.0161 7.6 56.2 Example 12Comparative 0.210 0.0153 7.1 61.2 Example 13 Comparative 0.250 0.01507.3 61.9 Example 14 Properties of magnetic metal particles containingiron as main component Examples X-ray Co Al Content of and crystallitecontent: content: rare earth Comparative size: D₁₁₀ Co/Fe Al/Fe element:Re/Fe Examples (nm) (atm %) (atm %) (atm %) Example 21 14.9 20 8 Ln: 8Example 22 15.5 10 10 Ln. 6 Example 23 14.1 10 7 Ln: 4 Example 24 15.530 12 Ln: 8 Example 25 15.2 30 7 Ln: 12 Example 26 15.9 30 10 Ln: 8Example 27 16.0 20 12 Ln: 8 Comparative 16.7 20 12 Ln: 8 Example 10Comparative 16.5 20 12 Ln: 8 Example 11 Comparative 16.7 20 12 Ln: 8Example 12 Comparative 14.0 5 3 B: 10 Example 13 Comparative 16.8 20 12Ln: 20 Example 14 Properties of magnetic metal particles containing ironas main component Examples Coercive Particle Saturation and force:coercive force magnetiza- Square- Comparative Hc distribution: tionness: Δσs Examples (Oe) SFDr σs (emu/g) σr/σs (%) Example 21 2,264 0.702146.3 0.538 9.7 Example 22 2,022 0.682 135.4 0.526 7.3 Example 23 1,9580.695 126.9 0.516 9.5 Example 24 2,241 0.692 146.6 0.541 6.8 Example 252,163 0.713 143.0 0.540 9.5 Example 26 2,068 0.715 151.2 0.529 9.5Example 27 2,019 0.718 153.4 0.518 9.7 Comparative 2,064 0.732 143.40.502 13.1 Example 10 Comparative 2,098 0.730 142.2 0.505 12.4 Example11 Comparative 1,943 0.761 135.3 0.501 12.4 Example 12 Comparative 1,8030.783 122.0 0.503 13.7 Example 13 Comparative 2,120 0.771 122.2 0.51012.1 Example 14 Sheet characteristics Examples Coercive Saturation andforce: magnetic flux Square- Comparative Hc density: ness: ΔBm Examples(Oe) Bm (G) Br/Bm SFD (%) Example 21 2,320 4,096 0.867 0.395 6.9 Example22 2,067 3,712 0.878 0.390 4.7 Example 23 1,995 3,376 0.875 0.392 6.8Example 24 2,291 4,130 0.873 0.394 4.8 Example 25 2,186 3,972 0.8700.401 7.1 Example 26 2,083 4,280 0.861 0.403 7.6 Example 27 2,036 4,2610.858 0.418 7.7 Comparative 2,061 3,988 0.840 0.458 12.0 Example 10Comparative 2,107 3,950 0.845 0.447 11.0 Example 11 Comparative 1,9213,690 0.833 0.465 11.4 Example 12 Comparative 1,847 3,186 0.864 0.48513.8 Example 13 Comparative 2,144 3,220 0.844 0.478 11.0 Example 14

[0320] TABLE 5 Properties of Goethite particles Average Particle Averagemajor size minor axial Standard distri- axial Starting diameterdeviation bution diameter particles (μm) (μm) (−) (μm) Goethite 0.1300.0246 0.189 0.0171 particles 1 Goethite 0.141 0.0271 0.192 0.0185particles 2 Goethite 0.120 0.0227 0.189 0.0162 particles 3 Goethite0.134 0.0265 0.198 0.0174 particles 4 Properties of Goethite particlesComposition ratio of seed portion Co content BET in seed Relationship ofspecific crystal to Co concentration Aspect surface Fe in seed in seedcrystal to Starting ratio area crystal Co concentration particles (−)(m²/g) (atm %) in particle Goethite 7.6 165.4 12.9 61.4 particles 1Goethite 7.6 169.5 13.0 61.9 particles 2 Goethite 7.4 158.8 12.9 61.4particles 3 Goethite 7.7 150.3  6.0 60.0 particles 4 Properties ofGoethite particles Starting Co content: Co/Fe Al content: Al/Feparticles (atm %) (atm %) Goethite 21 8 particles 1 Goethite 21 10particles 2 Goethite 21 7 particles 3 Goethite 10 4 particles 4

[0321] TABLE 6 Anti-sintering treatment Examples, Compound of rareReference earth element Cobalt Examples Re Co and content: content:Comparative Starting Re/Fe Co/Fe Examples particles Kind (atm %) (atm %)Example 28 Goethite Y(NO₃)₃ 8 9 particles 1 Example 29 Goethite Y(NO₃)₃6 4 particles 1 Example 30 Goethite Y(NO₃)₃ 6 — particles 2 Example 31Goethite Nd(NO₃)₃ 10 — particles 3 Reference Goethite Y(NO₃)₃ 8 10Example 1 particles 1 Reference Goethite Y(NO₃)₃ 6 — Example 2 particles2 Comparative Goethite Y(NO₃)₃ 8 9 Example 15 particles 1 ComparativeGoethite Y(NO₃)₃ 8 9 Example 16 particles 1 Comparative Goethite Y(NO₃)₃8 9 Example 17 particles 1 Comparative Goethite Y(NO₃)₃ 4 — Example 18particles 4 Examples, Reference Examples Conditions of conversion intohematite and Dehydrating Heating Comparative temperature Atmos-temperature Atmos- Examples (_C) phere (_C) phere Example 28 300 Air 600Air Example 29 280 Air 550 Air Example 30 320 Air 650 Air Example 31 320Air 650 Air Reference — — 600 Air Example 1 Reference — — 650 AirExample 2 Comparative 300 Air 750 Air Example 15 Comparative 300 Air 400Air Example 16 Comparative 300 N₂ 600 N₂ Example 17 Comparative 300 Air600 Air Example 18

[0322] TABLE 7 Examples, Properties of hematite particles ReferenceAverage Average Examples major Particle minor and axial Standard sizeaxial Comparative diameter deviation distribution diameter Examples (μm)(μm) (−) (μm) Example 28 0.120 0.0215 0.179 0.0164 Example 29 0.1150.0205 0.178 0.0162 Example 30 0.129 0.0222 0.172 0.0172 Example 310.106 0.0190 0.179 0.0149 Reference 0.114 0.0211 0.185 0.0163 Example 1Reference 0.124 0.0234 0.189 0.0180 Example 2 Comparative 0.117 0.03020.258 0.0191 Example 15 Comparative 0.125 0.0226 0.181 0.0160 Example 16Comparative 0.114 0.0298 0.261 0.0178 Example 17 Comparative 0.1150.0311 0.270 0.0177 Example 18 Examples, Reference Properties ofhematite particles Examples BET and Aspect specific Comparative ratiosurface area D₁₀₄ σs Examples (−) (m²/g) (_) (emu/g) Example 28 7.3 88.278 1.2 Example 29 7.1 85.4 75 0.8 Example 30 7.5 81.7 74 0.8 Example 317.1 90.3 70 0.7 Reference 7.0 84.4 91 3.0 Example 1 Reference 6.9 76.987 3.1 Example 2 Comparative 6.1 62.2 103 16.8 Example 15 Comparative7.8 151.6 38 0.2 Example 16 Comparative 6.4 65.5 105 15.1 Example 17Comparative 6.5 58.6 90 2.8 Example 18 Examples, Properties of hematiteparticles Reference Content of Examples rare earth and Co content: Alcontent: element: Comparative Co/Fe Al/Fe Re/Fe Examples (atm %) (atm %)(atm %) Example 28 30 8 8 Example 29 25 8 6 Example 30 21 10 6 Example31 21 7 10 Reference 30 8 8 Example 1 Reference 21 10 5 Example 2Comparative 30 8 8 Example 15 Comparative 30 8 8 Example 16 Comparative30 8 8 Example 17 Comparative 10 4 4 Example 18

[0323] TABLE 8 Examples, Properties of magnetic Reference Heat-spindle-shaped metal Examples reduction particles and Reducing Averagemajor Standard Comparative temperature axial deviation Examples (_C)diameter (μm) (μm) Example 32 600 0.101 0.0158 Example 33 600 0.0980.0151 Example 34 600 0.112 0.0171 Example 35 600 0.091 0.0137 Reference600 0.100 0.0168 Example 3 Reference 600 0.110 0.0184 Example 4Comparative 600 0.097 0.0221 Example 19 Comparative 600 0.081 0.0273Example 20 Comparative 600 0.098 0.0228 Example 21 Comparative 600 0.1060.0263 Example 22 Examples Properties of magnetic spindle-shaped metalparticles Reference particle Average BET Examples size minor specificand distri- axial Aspect surface Comparative bution diameter ratio areaExamples (−) (um) (−) (m²/g) Example 32 0.156 0.0146 6.9 48.6 Example 330.154 0.0151 6.5 49.8 Example 34 0.153 0.0165 6.8 47.1 Example 35 0.1510.0140 6.5 52.3 Reference 0.168 0.0154 6.5 47.2 Example 3 Reference0.167 0.0169 6.5 46.8 Example 4 Comparative 0.228 0.0167 5.8 38.6Example 19 Comparative 0.337 0.0169 4.8 52.6 Example 20 Comparative0.233 0.0166 5.9 45.5 Example 21 Comparative 0.248 0.0171 6.2 40.3Example 22 Properties of magnetic spindle-shaped Examples, metalparticles Reference Content of Examples Co Al rare earth and content:content: element: Comparative D₁₁₀ Co/Fe Al/Fe Re/Fe Examples (nm) (atm%) (atm %) (atm %) Example 32 15.8 30 8 3 Example 33 15.5 25 8 6 Example34 16.0 21 10 6 Example 35 14.8 21 7 10 Reference 15.6 30 8 8 Example 3Reference 16.1 21 10 6 Example 4 Comparative 16.3 30 8 8 Example 19Comparative 16.7 30 8 8 Example 20 Comparative 15.9 30 8 8 Example 21Comparative 16.0 10 4 4 Example 22 Examples, Properties of magneticspindle-shaped Reference metal particles Examples Coercive Saturationand force: magnetization Square- Comparative Hc σs Δσs ness: Examples(Oe) (emu/g) (%) σr/σs Example 32 2,289 142.2 6.8 0.534 Example 33 2,280138.9 6.5 0.532 Example 34 2,207 141.3 5.7 0.529 Example 35 2,276 136.17.2 0.530 Reference 2,242 145.3 8.3 0.530 Example 3 Reference 2,152142.5 7.8 0.521 Example 4 Comparative 2,033 148.8 13.0 0.509 Example 19Comparative 1,932 131.4 14.4 0.498 Example 20 Comparative 2,200 146.811.2 0.516 Example 21 Comparative 1,904 138.3 10.4 0.501 Example 22

[0324] TABLE 9 Examples, Properties of magnetic coating film ReferenceCoercive Saturation Examples and force: magnetic flux Square-Comparative Hc density: ness: ΔBm Examples (Oe) Bm (G) Br/Bm SFD (%)Example 32 2,332 3,932 0.870 0.387 5.0 Example 33 2,319 3,850 0.8750.380 4.3 Example 34 2,254 3,913 0.869 0.387 3.9 Example 35 2,321 3,7820.865 0.384 5.4 Reference 2,289 4,022 0.870 0.393 6.1 Example 3Reference 2,193 3,940 0.868 0.398 6.0 Example 4 Comparative 2,013 4,1330.843 0.469 12.4 Example 19 Comparative 1,903 3,638 0.821 0.512 13.8Example 20 Comparative 2,167 4,061 0.840 0.488 10.5 Example 21Comparative 1,944 3,847 0.836 0.501 10.6 Example 22

[0325] TABLE 10 Properties of Goethite particles Average ParticleAverage major size minor axial Standard distri- axial Starting diameterdeviation bution diameter particles (μm) (μm) (−) (μm) Goethite 0.1260.0238 0.189 0.0175 particles 5 Goethite 0.138 0.0258 0.187 0.0177particles 6 Properties of spindle-shaped goethite particles Compositionratio of seed portion Co content BET in seed Relationship of specificcrystal to Co concentration Aspect surface Fe in seed in seed crystal toStarting ratio area crystal Co concentration particles (−) (m²/g) (atm%) in particle Goethite 7.2 193.2 15.4 61.6 particles 5 Goethite 7.8170.3 12.9 61.4 particles 6 Properties of goethite particles Starting Cocontent: Co/Fe Al content: Al/Fe particles (atm %) (atm %) Goethite 2510 particles 5 Goethite 21  8 particles 6

[0326] TABLE 11 Anti-sintering treatment Compound of rare earth elementRe content: Hematite Starting Re/Fe particles particles Kind (atm %)Hematite Goethite Y(NO₃)₃ 8 particles 1 particles 5 Hematite GoethiteNd(NO₃)₃ 10 particles 2 particles 5 Hematite Goethite Y(NO₃)₃ 6particles 3 particles 6 Hematite Goethite Nd(NO₃)₃ 8 particles 4particles 6 Anti-sintering Conditions of treatment conversion intoCobalt compound hematite Co Dehydrating Heating content: tempera-tempera- Hematite Co/Fe ture ture particles Kind (atm %) (_C) (_C)Hematite CoSO₄ 10 300 600 particles 1 Hematite CoSO₄ 10 330 650particles 2 Hematite CoSO₄ 9 300 550 particles 3 Hematite Co(C₂H₃O₂)₂ 4330 600 particles 4

[0327] TABLE 12 Properties of hematite particles Average ParticleAverage major axial Standard size minor axial Hematite diameterdeviation distribution diameter particles (μm) (μm) (−) (μm) Hematite0.117 0.0205 0.175 0.0167 particles 1 Hematite 0.113 0.0192 0.170 0.0169particles 2 Hematite 0.129 0.0232 0.180 0.0170 particles 3 Hematite0.125 0.0220 0.176 0.0171 particles 4 Properties of hematite particlesBET Co Al Content of Aspect specific content: content: rare earthHematite ratio surface area Co/Fe Al/Fe element: particles (−) (m²/g)(atm %) (atm %) Re/Fe (atm %) Hematite 7.0 92.3 35 10 8 particles 1Hematite 6.7 84.7 35 10 10 particles 2 Hematite 7.6 90.1 30 8 6particles 3 Hematite 7.3 88.2 25 8 8 particles 4

[0328] TABLE 13 Hematite Examples particles or and Goethite Reductionconditions Comparative particles Bed height Kind of Examples used (cm)heating gas Example 36 Hematite 27 N₂ particles 1 Example 37 Hematite 30N₂ particles 2 Example 38 Hematite 10 N₂ particles 3 Example 39 Hematite20 N₂ particles 4 Example 40 Goethite 15 N₂ particles 5 ComparativeHematite 27 H₂ Example 23 particles 1 Comparative Hematite 27 N₂ Example24 particles 1 Comparative Hematite 30 N₂ Example 25 particles 2Comparative Hematite 35 N₂ Example 26 particles 1 Comparative Hematite30 N₂ Example 27 particles 2 Comparative Hematite 10 N₂ Example 28particles 3 Comparative Hematite 8 N₂ Example 29 particles 3 ExamplesReduction conditions and Linear Reducing Comparative Kind of velocitytemperature Examples reducing gas (cm/s) (_C) Example 36 H₂ 100 550Example 37 H₂ 150 650 Example 38 H₂ 50 450 Example 39 H₂ 80 600 Example40 H₂ 100 500 Comparative H₂ 100 550 Example 23 Comparative H₂ 30 550Example 24 Comparative H₂ 170 650 Example 25 Comparative H₂ 50 550Example 26 Comparative H₂ 50 720 Example 27 Comparative H₂ 50 380Example 28 Comparative H₂ 7 400 Example 29

[0329] TABLE 14 Properties of magnetic spindle-shaped metal particlesAverage Average Examples major Particle minor and axial Standard sizeaxial Comparative diameter deviation distribution diameter Examples (μm)(μm) (−) (μm) Example 36 0.100 0.0155 0.150 0.0154 Example 37 0.0950.0138 0.145 0.0158 Example 38 0.114 0.0177 0.155 0.0163 Example 390.109 0.0166 0.152 0.0163 Example 40 0.101 0.0148 0.147 0.0153Comparative 0.093 0.0190 0.204 0.0169 Example 23 Comparative 0.0900.0190 0.211 0.0170 Example 24 Comparative Unmeasurable because moldedparticles were Example 25 scattered and dissipated Comparative 0.0940.0202 0.215 0.0157 Example 26 Comparative 0.070 0.0176 0.251 0.0233Example 27 Comparative 0.118 0.0180 0.153 0.0164 Example 28 Comparative0.105 0.0218 0.208 0.0219 Example 29 Properties of magneticspindle-shaped metal particles Content of Examples Co Al rare earth andAspect content: content: element: Comparative ratio Co/Fe Al/Fe Re/FeExamples (−) (atm %) (atm %) (atm %) Example 36 6.5 35 10 8 Example 376.0 35 10 10 Example 38 7.0 30 8 6 Example 39 6.7 25 8 8 Example 40 6.635 10 8 Comparative 5.5 35 10 8 Example 23 Comparative 5.3 35 10 8Example 24 Comparative Unmeasurable because molded Example 25 particleswere scattered and dissipated Comparative 6.0 35 10 8 Example 26Comparative 3.0 35 10 10 Example 27 Comparative 7.2 30 8 6 Example 28Comparative 4.8 30 8 6 Example 29 Properties of magnetic spindle-shapedExamples metal particles and BET specific Coercive Comparative surfacearea D₁₁₀ force: Hc Examples (m²/g) (_) (Oe) Example 36 53.8 148 2,315Example 37 44.3 155 2,355 Example 38 58.7 140 2,222 Example 39 50.2 1502,295 Example 40 57.2 142 2,331 Comparative 42.3 163 1,985 Example 23Comparative 39.1 167 1,958 Example 24 Comparative Unmeasurable becausemolded Example 25 particles were scattered and dissipated Comparative40.3 165 1,987 Example 26 Comparative 32.4 171 1,870 Example 27Comparative 68.4 118 1,872 Example 28 Comparative 38.3 166 1,992 Example29 Properties of magnetic spindle-shaped metal particles ExamplesSaturation and magnetization: Comparative σs R/S Δσs Examples (emu/g)(−) (%) Example 36 144.1 0.537 8.8 Example 37 152.5 0.542 9.9 Example 38140.2 0.530 7.0 Example 39 147.0 0.532 8.5 Example 40 142.7 0.541 7.1Comparative 143.9 0.511 11.2 Example 23 Comparative 145.0 0.510 12.3Example 24 Comparative Unmeasurable because molded Example 25 particleswere scattered and dissipated Comparative 144.3 0.512 11.5 Example 26Comparative 145.7 0.497 10.0 Example 27 Comparative 110.5 0.513 7.1Example 28 Comparative 143.5 0.509 12.7 Example 29

[0330] TABLE 15 Magnetic properties of lower bed portion (3 cm)Saturation Examples Coercive magneti- and force: zation: Comparative Hcσs Squarenes D₁₁₀ Examples (Oe) (emu/g) s (−) (_) Example 36 2,332 143.50.542 146 Comparative 2,032 138.8 0.518 155 Example 23 Example 36 2,306145.0 0.539 149 Comparative 1,880 145.5 0.500 170 Example 23

[0331] TABLE 16 Properties of magnetic coating film Saturation Examplesmagnetic flux Square- and Coercive density: ness: Comparative force: BmBr/Bm SFD ΔBm Examples Hc (Oe) (Gauss) (−) (−) (%) Example 36 2,3364,130 0.870 0.385 6.7 Example 37 2,380 4,310 0.873 0.378 8.1 Example 382,260 4,000 0.877 0.393 5.1 Example 39 2,310 4,200 0.875 0.390 6.3Example 40 2,375 4,100 0.878 0.375 5.0 Comparative 2,003 4,150 0.8170.545 9.5 Example 23 Comparative 1,960 4,200 0.811 0.551 10.1 Example 24Comparative Unmeasurable because molded particles Example 25 werescattered and dissipated Comparative 2,015 4,100 0.805 0.566 9.1 Example26 Comparative 1,875 4,200 0.723 0.885 8.0 Example 27 Comparative 1,9233,150 0.803 0.588 5.5 Example 28 Comparative 2,001 4,080 0.785 0.62510.2 Example 29

What is claimed is:
 1. Spindle-shaped goethite particles containing Coof 8 to 45 atm %, calculated as Co, based on whole Fe, Al of 5 to 20 atm%, calculated as Al, based on whole Fe, and having an average majoraxial diameter of 0.05 to 0.18 μm, each of said spindle-shaped goethiteparticles comprising a seed portion and a surface layer portion, theweight ratio of said seed portion to said surface layer portion being30:70 to 80:20 and the relationship of the Co concentration of the seedportion with that of the goethite particle being 50 to 95:100 when theCo concentration of the goethite particle is 100, and said aluminumexisting only in said surface layer portion.
 2. Spindle-shaped goethiteparticles according to claim 1, which further have an average minoraxial diameter of 0.010 to 0.025 μm, an aspect ratio (average majoraxial diameter/average minor axial diameter) of 4:1 to 8:1, a particlesize distribution (standard deviation/average major axial diameter) ofnot more than 0.24 and a BET specific surface area of 100 to 250 m²/g.3. Spindle-shaped goethite particles according to claim 1, which containcobalt of 10 to 40 atm %, calculated as Co, based on whole Fe, andaluminum of 6 to 15 atm %, calculated as Al, based on whole Fe, and havean average major axial diameter of 0.05 to 0.16 μm and an average minoraxial diameter of 0.010 to 0.023 μm, and wherein the weight ratio ofsaid seed portion to said surface layer portion is 40:60 to 70:30, andthe relationship of the Co concentration of the seed portion with thatof the goethite particle being 60 to 90:100 when the Co concentration ofthe goethite particle is
 100. 4. Spindle-shaped goethite particlesaccording to claim 1, which contain cobalt of more than 20 atm % and notmore than 45 atm %, calculated as Co, based on whole Fe, and aluminum of5 to 15 atm %, calculated as Al, based on whole Fe, and have an averagemajor axial diameter of 0.05 to 0.17 μm, an average minor axial diameterof 0.010 to 0.025 μm.
 5. Spindle-shaped goethite particles according toclaim 1, which further have an X-ray crystallite size ratio (D₀₂₀/D₁₁₀)of not less than 2.0:1.
 6. Spindle-shaped hematite particles containingcobalt of 8 to 45 atm %, calculated as Co, based on whole Fe, aluminumof 5 to 20 atm %, calculated as Al, based on whole Fe, and a rare earthelement of 1 to 15 atm %, calculated as rare earth element, based onwhole Fe, and having an average particle size of 0.05 to 0.17 μm, eachof said spindle-shaped hematite particles comprising a seed portion, anintermediate layer portion and an outer layer portion, the weight ratioof said seed portion to said intermediate layer portion being 30:70 to80:20 and the relationship of the Co concentration of the seed portionwith that of the hematite particle being 50 to 95:100 when the Coconcentration of the hematite particle is 100, said aluminum existingonly in said intermediate layer portion and said rare earth elementexisting in said outer layer portion.
 7. Spindle-shaped hematiteparticles according to claim 6, which have an average major axialdiameter of 0.05 to 0.14 μm, an aspect ratio (average major axialdiameter/average minor axial diameter) of 4:1 to 8:1, a crystallite sizeD₁₀₄ of 50 to 80 Å, a saturation magnetization σs of 0.5 to 2 emu/g, andcontain cobalt of more than 20 atm % and not more than 45 atm %,calculated as Co, based on whole Fe, aluminum of 5 to 15 atm %,calculated as Al, based on whole Fe, and a rare earth compound of 5 to15 atm %, calculated as rare earth element, based on whole Fe. 8.Spindle-shaped hematite particles according to claim 6, which furtherhave an average minor axial diameter of 0.010 to 0.025 μm, an aspectratio (average major axial diameter/average minor axial diameter) of 4:1to 8:1, a particle size distribution (standard deviation/average majoraxial diameter) of not more than 0.22 and a BET specific surface area of30 to 150 m²/g.
 9. Spindle-shaped hematite particles according to claim6, which contain cobalt of 10 to 40 atm %, calculated as Co, based onwhole Fe, aluminum of 6 to 15 atm %, calculated as Al, based on wholeFe, and a rare earth element of 4 to 12 atm %, calculated as rare earthelement, based on whole Fe, and wherein the weight ratio of said seedportion to said intermediate layer portion is 40:60 to 70:30; and therelationship of the Co concentration of the seed portion with that ofthe hematite particle being 60 to 90:100 when the Co concentration ofthe hematite particle is
 100. 10. Spindle-shaped hematite particlesaccording to claim 6, wherein said rare earth element is at least oneelement selected from the group consisting of scandium, yttrium,lanthanum, cerium, praseodymium, neodymium and samarium. 11.Spindle-shaped hematite particles according to claim 7, which furtherhave an average minor axial diameter of 0.010 to 0.22 μm, a particlesize distribution (standard deviation/average major axial diameter) ofnot more than 0.20 and a BET specific surface area of 30 to 150 m²/g.12. Spindle-shaped hematite particles according to claim 7, whichfurther have an average major axial diameter of 0.05 to 0.13 μm, anaspect ratio of 4:1 to 7.5:1, an X-ray crystallite size D₁₀₄ of 50 to 78Å and a saturation magnetization σs of 0.5 to 1.5 emu/g, and containcobalt of 21 to 40 atm %, calculated as Co, based on whole Fe, aluminumof 6 to 14 atm %, calculated as Al, based on whole Fe, and a rare earthelement of 5 to 12 atm %, calculated as rare earth element, based onwhole Fe.
 13. Magnetic spindle-shaped metal particles containing iron asa main component, which contain cobalt of 8 to 45 atm %, calculated asCo, based on whole Fe, aluminum of 5 to 20 atm %, calculated as Al,based on whole Fe, and a rare earth element of 1 to 15 atm %, calculatedas rare earth element, based on whole Fe, and have an average majoraxial diameter of 0.05 to 0.15 μm.
 14. Magnetic spindle-shaped metalparticles containing iron as a main component according to claim 13,which contain cobalt of more than 20 atm % and not more than 45 atm %,calculated as Co, based on whole Fe, aluminum of 5 to 15 atm %,calculated as Al, based on whole Fe, and a rare earth element of 5 to 15atm %, calculated as rare earth element, based on whole Fe, and have anaverage major axial diameter of 0.05 to 0.14 μm, an aspect ratio(average major axial diameter/average minor axial diameter) of 4:1 to8:1, an X-ray crystallite size D₁₁₀ of 12.0 to 17.0 nm, a coercive forceof 2,000 to 2,500 Oe and a saturation magnetization σs of 130 to 160emu/g.
 15. Magnetic spindle-shaped metal particles containing iron as amain component according to claim 13, which further have an averageminor axial diameter of 0.010 to 0.022 μm, an aspect ratio (averagemajor axial diameter/average minor axial diameter) of 4:1 to 7:1, aparticle size distribution (standard deviation/average major axialdiameter) of not more than 0.20, a BET specific surface area of 35 to 65m²/g, a coercive force of 1,800 to 2,500 Oe and a saturationmagnetization σs of 110 to 160 emu/g.
 16. Magnetic spindle-shaped metalparticles containing iron as a main component according to claim 15,which further have a particle coercive force distribution (SFDr) of notmore than 0.72, an X-ray crystallite size D₁₁₀ of 12.0 to 17.0 nm and achange in a saturation magnetization σs with passage of time of not morethan 15% as an absolute value.
 17. Magnetic spindle-shaped metalparticles containing iron as a main component according to claim 14,which further have an average minor axial diameter of 0.010 to 0.020 μm,a particle size distribution (standard deviation/average major axialdiameter) of not more than 0.18, a BET specific surface area of 35 to 65m²/g and a change in a saturation magnetization σs with passage of timeof not more than 10% as an absolute value.
 18. A process for producingthe spindle-shaped goethite particles, comprising: aging a watersuspension containing an Fe²⁺-containing precipitate produced byreacting a mixed aqueous alkali solution comprising an aqueous alkalicarbonate solution and an aqueous alkali hydroxide solution, with anaqueous ferrous salt solution, in a non-oxidative atmosphere; conductingthe oxidation reaction by passing an oxygen-containing gas through thewater suspension, thereby producing spindle-shaped goethite seed crystalparticles; and passing again an oxygen-containing gas through theresultant water suspension containing both said Fe²⁺-containingprecipitate and said spindle-shaped goethite seed crystal particles toconduct the oxidation reaction of the water suspension, thereby growinga goethite layer on a surface of each spindle-shaped goethite seedcrystal particle, upon the production of said spindle-shaped goethiteseed crystal particles, a Co compound being added in an amount of 8 to45 atm %, calculated as Co, based on whole Fe, to said water suspensioncontaining the Fe²⁺-containing precipitate during the aging treatmentbefore initiation of the oxidation reaction, thereby oxidizing 30 to 80%of whole Fe²⁺, and upon the growth of said goethite layer, a linearvelocity of said oxygen-containing gas passing through said watersuspension containing both the Fe²⁺-containing precipitate and thespindle-shaped goethite seed crystal particles, being adjusted to notless than two times that of the oxygen-containing gas passing throughthe water suspension containing the Fe²⁺-containing precipitate upon theproduction of the goethite seed crystal particles, and an Al compoundbeing added in an amount of 5 to 20 atm %, calculated as Al, based onwhole Fe.
 19. A process for producing spindle-shaped hematite particles,comprising: treating said spindle-shaped goethite particles obtained inclaim 18 with an anti-sintering agent comprising a rare earthelement-containing compound; and then heat-treating the spindle-shapedgoethite particles at 400 to 850° C. in a non-reducing atmosphere.
 20. Aprocess for producing magnetic spindle-shaped metal particles containingiron as a main component, comprising: treating said spindle-shapedgoethite particles obtained in claim 18 with an anti-sintering agentcomprising a rare earth element-containing compound; and thenheat-reducing said spindle-shaped goethite particles at 400 to 700° C.in a reducing atmosphere.
 21. A process for producing magneticspindle-shaped metal particles containing iron as a main component,comprising: treating said spindle-shaped goethite particles obtained inclaim 18 with an anti-sintering agent comprising a rare earthelement-containing compound; heat-treating said spindle-shaped goethiteparticles at 400 to 850° C. in a non-reducing atmosphere; and thenheat-reducing said treated particles at 400 to 700° C. in a reducingatmosphere.
 22. A process for producing magnetic spindle-shaped metalparticles containing iron as a main component, comprising: heat-reducingsaid spindle-shaped hematite particles obtained in claim 19 at 400 to700° C. in a reducing gas atmosphere.
 23. A process for producingmagnetic spindle-shaped metal particles containing iron as a maincomponent and suitable for magnetic recording, comprising: chargingspindle-shaped goethite particles containing cobalt of 20 to 45 atm %,calculated as Co, based on whole Fe and having a major axial diameter of0.05 to 0.15 μm, or spindle-shaped hematite particles obtained byheat-dehydrating said goethite particles, as starting particles, into afixed-bed reducing reactor to form a fixed-bed having a height of notmore than 30 cm; elevating the temperature of said starting particles to400 to 700° C. in an inert gas atmosphere; replacing the inert gasatmosphere with a reducing gas atmosphere; and reducing saidspindle-shaped goethite particles or spindle-shaped hematite particleswith a reducing gas fed at a linear velocity of 40 to 150 cm/s, attemperature of 400 to 700° C.